The objective of this study was to collect information related to prevention of the degradation of salt affected sandy soils by new method of reforestation (different tree species plantation) on the upper Sieo watershed area Amphur Borabue, Mahasarakam province. The experiment was setting for 5 years. The 7.2 hectares study area had 3-5% sloping degree with salt affected soil at the lowland. The area was separated to 3 terrain levels as high, middle and low terrain respectively. Each terrain was planted with 24 tree species at 2x2 m spacing to decrease the level of underground water which is one of the cause of saline soil formation and to reduce the surface soil erosion. The data were collected on soil sediment, plant nutrients lost with surface soil erosion at before and after tree planting, changes in amount of organic matter and cation exchange capacity of the surface soil (0-10 cm), level of underground water and its electroconductivity changes at the lowland area affected by salt. The results obtained after two years of tree growing showed that the amounts of soil sediment loss in the high and middle terrain were reduced from 59.00 and 64.75 kg/hectare in the year 2003 to 37.31 and 31.06 kg/hectare in the year 2004, respectively. On the other hand, the amount of soil sediment on the low terrain was increased from 57.31 kg/hectare in the year 2003 to 75.75 kg/hectare in the year 2004 indicating the move of soil sediment from the high terrain to accumulate on the low terrain. The amounts of N and P in the soil sediment on the high and middle terrains had tendencies in decreasing from 0.032 and 0.002% in the year 2003 to 0.027 and 0.001% in the year 2004, respectively. But the amount of K in soil sediment was on the other hand, increased from 0.03% in the year 2003 to 1.11% in the year 2004. In the middle terrain the amounts of N, P and K were increased from 0.021, 0.001 and 0.024% in the year 2003 to 0.041, 0.003 and 1.275% in the year 2004, respectively. That was the same situation on the low terrain where the amounts of N, P and K were increased from 0.028, 0.001 and 0.027% in the year 2003 to 0.038, 0.002 and 1.034% in the year 2004, respectively. Meanwhile the changes amount of organic matter of the surface soil (0-10 cm) within 2 years (2002-2004) was in creased from 0.20% to 0.30% on the high terrain, from 0.28% to 0.33% at middle terrain and from 0.32% to 0.37% at low terrain respectively. The values of cation exchange capacity of surface soil (0-10 cm) were change from 1.18 meq/100g soil to 2.18 meq/100g soil on the high terrain, from 1.94 meq/100g soil to 2.42 meq/100g soil on the middle sloping area and from 1.57 meq/100g soil to 2.06 meq/100g soil on the low terrain respectively. The level of underground water measured in the dry season was lower down from 152 cm in the year 2003 to 217 cm depth in the year 2004 while it was down from 40 cm in year 2003 to 120 cm in year 2004 when measured in the rainy season. Meanwhile its electroconductivity of underground water was ranged between 3.0-3.5 mS/cm in the dry season and 1.3-2.7 mS/cm in the rainy season.