The economic benefit of sediment reduction by pine forest and black acacia forest at different age classes in Miyun reservoir watershed was studied using the approach of relocation cost based on the field survey from 1996 to 1999. The results show that if the transportation of eroded soil into Miyun reservoir was estimated as 100%, the economic benefit of sediment reduction by pine forest would be 59~164 RMB yuan/ha(RMB yuan = Renminbi yuan, or yuan) per year (US$ 1=8.27 RMB yuan during the period of research), and 58~134 yuan/ha by black acacia forest. The average economic benefit of sediment reduction by forest is 130 yuan/ha per year. If r (the rate at which the eroded soil will enter a reservoir) varies, the economic benefits of sediment reduction also change. To estimate the economic benefit of sediment reduction more accurately, r should be measured from the field test. The results from this study provide information to help assess the feasibility of the environmental project in this area. The economic benefit of sediment reduction by pine forest and black acacia forest at different age classes in Miyun reservoir watershed was studied using the approach of relocation cost based on the field survey from 1996 to 1999. The results show that if the transportation of eroded soil into Miyun reservoir was estimated as 100%, the economic benefit of sediment reduction by pine forest would be 59 ~ 164 RMB yuan / ha (RMB yuan = Renminbi yuan, or yuan) per year (US $ 1 = 8.27 RMB yuan during the period of the average economic benefit of sediment reduction by forest is 130 yuan / ha per year. If r (the rate at which the eroded soil will enter a reservoir) varies, the economic benefit of sediment reduction also change. To estimate the economic benefit of sediment reduction more accurately, r should be measured from the field test. The results from this study provide information to help assess the feasibility of the environmental project in this area.