Hydro-power plants, as a part of infrastructure projects, play an important role in the economic-social development of countries. Since a large amount of investment is needed for construction of these power plants, which appeared to be an obstacle in these developments, however it is possible to finance these infrastructure plants by assigning these affairs to private sectors by using build operate transfer (BOT) method, which is quite well-known all around the world. This paper reviews the structure of BOT contracts and through an economic evaluation based on different percentage of investments of private sector in providing the expenses of small and medium hydro-power plants (S&M-HPP) (e.g. MHPP in "Bookan, Iran" and SHPP in "Nari, Iran"), demonstrates that by increasing the percentage the share of the private sector in the investment, the economic indices B/C and NPV improve substantially. (c) 2006 Elsevier Ltd. All rights reserved.