This study separated the effects of climate change and human activities on river flow in the Heihe watershed (a tributary of Jinghe river) during 1972-2000 using statistical methods of the Mann-Kendall trend tests and double mass curve. Results showed that climate tended to become warmer and drier while river flow became lower for the period. The whole study period could be divided into two sub-periods (1972-1992 and 1993-2000) by the year when river flow underwent abrupt change. Compared with the former period, precipitation decreased by 75.4 mm and temperature increased by 0.5 ℃ while rive flow decreased by 29 mm in the latter period. Both climate change and human activities caused a decrease in river flow, with contributions being 24% for the former and 76% for the latter. Overall, the results indicated that human activities were the main driving forces for the decrease of river flow. However, the impacts of climate change can not be ignored, and the effects should be taken into account when carrying out ecological construction and water resources management in the Loess Plateau.