Iron-manganese nodules are dominant redoximorphic features in anthraquic and seasonally fluctuation high ground water table rice-growing Ultisols in Taiwan. The quantities and characteristics of these nodules were mainly affected by changing of the conditions in the water regime. The objective of this study was to determine the chemical and physical properties of the Fe-Mn nodules with different sizes and to propose the formation mechanism under different soil water regimes. Three Fe-Mn rich nodules from Ultisols with plinthite along a hydrosequence in northern Taiwan(the Chungli terraces)were selected. These were the Hukou soil (Plinthic Paleaquult), Chuwei soil (Typic Plinthaquult) and Luchu soil (Typic Plinthaquult) with elevations of 20 to 30m, respectively. The measurements of the ground water table, soil saturation and soil redox potential (Eh) are combined with measurements of physical and chemical characteristics and Fe-Mn nodules with different sizes to understand the formation of these redoximorphic features in the soils. For the depth of 50cm from the soil surface, we determined that the pedon with the highest reduction (70%/year), Hukou soil, had the lowest amounts of Fe-Mn nodules (9 kg/150 cm/m2). The Chuwei soil was the moderately reduced pedon (40%/year) that had the largest number of Fe-Mn nodules (740 kg/160 cm/m2).Accordingly, the period of moderate reduction (about 40%) was assumed to be the most suitable condition for the formation of Fe-Mn nodules in the study area. Amorphous materials (Feo, Mno, and Alo), crystalline materials (Fed, Mnd, and Ald), and total contents (Fet and Mnt) of Fe, Mn and Al were studied in different sizes of 2-5, 5-10, 10-20, and >20mm to determine the formation of nodules under different water regime conditions. The results revealed that Fed and Fet decreased with increasing nodule sizes. A trend was not determined in any of the fractions of Mn in all three pedons. The crystallinity index ((Fed-Feo)/Fet) of different size nodules ranged from 0.5 to 1. There was a significant negative relation between the crystallinity index and the reduced duration of a year (r=0.47**, p<0.01). This indicated that the degree of crystallinity of the nodules decreased with the increasing reduction duration of the horizons. All nodules with different sizes have a similar Fe activity index (Feo/Fed). There is a trend for the Feo/Fed ratio slightly increasing with the nodule size, and the largest nodule (> 20 mm) having a significantly lower bulk density compared to nodules of other sizes. These evidences implied to the poor crystallinity of iron and the continuing accretion of smaller nodules to the larger ones. The micromorphological features of Fe-Mn nodules with different sizes and water regime conditions were also studied. These features indicated that the majority of the nodules were formed in situ with diffuse boundaries. Moreover, the amount of the Mn nodules of different sizes was almost highest in the horizon with the strongest reduction conditions, such as the Ap horizon of Luchu soil, and the Ap and AB horizons of the Chuwei soils. In these horizons, the reduced Mn was transported and precipitated on the surface of the coarser minerals such as quartz. On the other hand, the reduced Fe or Mn, probably, repeatedly precipitated on the surfaces of the minerals or was filling their cavities, and growing larger. These features also indicated that the concentric nodules only present in the Luchu soil were smaller than 0.5 mm due to the clay loam soil texture. There are fewer, smaller and poorly developed nodules in the Hukou pedon, due to the long-term reduced conditions and higher organic matter contents. According to the results, we propose a consecutive three stage formation process for the Fe-Mn nodules in these Ultisols with plinthites as in the following: a) the transportation of the mobile reduced Mn to mineral surfaces, b) the re-oxidization to poor crystalline Mn oxide in the initial stage of nodule formation and the continuing Fe or Mn precipitation on or filling the Mn nodules as more crystalline Fe forms and finally c) the continued growth of these smaller nodules adding more poorly crystalline Fe to form a bigger volume of Fe-Mn nodules in the plinthite of the three studied soils. Key words: Iron-manganese nodules, redoximorphic features, Ultisols, plinthite, crystalline index, reduced condition, rice-growing soil