A study of the variation of organic matter content in stream water and in sediment of Moei River basin in Tak Province was conducted. Samples were collected at five different classes of watershed and in twenty sub-basins during November 1990 - September 1991. The results revealed that the quantity of organic matter in stream water ranged from 0.84 - 236.10 mg/l with the average of 11.76 mg/l. The amount of organic matter in sediment varied from 0.10 - 8.40% with the average of 1.52%. The results also revealed that the quantity of organic matter in stream water and in sediment were not in agreement with the classification of watershed. The average organic matter content of stream water in summer, winter and raing season were 16.31, 9.66 and 9.32 mg/l, respectively and in sediment were 2.10, 1.41 and 1.06%, respectively. The factors that affect the quantity of organic matter in both stream water and sediment depended on the amount of water, stream flow rate, landuse, efficiency of drainage system, size of watershed and type of soil texture.