Environmental Impact Assessment (EIA) is essentially a preventive process by which costly and irreversible mistakes could be easily avoided during the project development and implementation. It serves as an important tool in environmental planning through identification, prediction and evaluation of the possible impacts and mitigation of the development projects. This study examines the effectiveness of the environmental impacts and adopted mitigation measures in the field of biological environment of a completed (Kali Gandaki A Hydroelectric Project) and an under-construction (Middle Marsyangdi Hydroelectric Project) dam project located in the similar climatic and topographical environments of middle hill region of Nepal through a qualitative manner.This evaluation was done basically to check back and examine how well the prescribed environmental instruments have worked in these projects.