Small hydropower construction investment is developing in Vietnam, most of these small hydroelectric stations working on the same river system and built under BOT form. Working mode of the hydroelectric stations at upstream directly affect the energy efficiency and economy of hydroelectric stations at downstream. The affects on the power of dry season, component powers depending on capacity and regulatory forms of upstream hydroelectric stations. The larger reservoirs of hydroelectric stations at upstream, the long-term regulation affect the higher influence. Therefore, in the simulation calculation should coordinate inter-reservoir operation instead of operating independently in order to bring greater economic efficiency.