The purpose of this study is to compare decomposition characteristics of a number of organic inputs being used in organic rice cultivation. An incubation experiment of paddy soil with application of eight organic matters, including organic by-products (rice bran, rape seed cake), green manures (hairy vetch, chinese milk vetch, rye), composts (rice straw compost, pig manure compost), and plant residues (rice straw), were conducted. Additional eight treatments of without organic matter (only soil), chemical fertilizer (urea, urea and rice straw compost), and mixtures of rice straw and hairy vetch, of which C:N ratio were adjusted to 15, 20, 25, 30, and 40 respectively, were tested. Organic inputs and paddy soil mixtures were incubated in test tube in submerged condition with temperature of 20'C, 25'C, and 30'C. Mineralized nitrogen, microbial biomass C and N were determined peoridically during incubation from 1 to 90 days. Mineralization potential and rate constant were estimated by kinetic models. Green manures and by-products were higher in mineralization potential while rye, composts, and plant residues were relatively low, indicating C:N and chemical composition are closely related to mineralization properties. Increased C:N ratio reduced mineralization potential of input materials. Mineralization rate were increased in higher temperature conditions. However, its magnitude were somewhat different among treatments. Microbial biomass C showed fluctuating patterns but significantly different in variety of C:N ratio and temperature conditions.