One of the best practical methods to understand ecological status of a water body and determine impacts of human intervention in reducing water quality is using benthic macroinvertebratesas assessment tools for monitoring their biological integrity and health. The aim of this study is determining the water quality of Gahar Lake by using of Hilsenhoff Family Biotic index (HFBI) index and identify the society of macroinvertebrates in level of the family. This research was achieved in 2011along 3 season sampling in four different station in the small Gahar (first station), the input of small Gahar to large Gahar (second station), the centre of great Gahar (third station) and the of output great Gahar (fourth station) was taken and it has identified for family level in the laboratory. In which they have most variety and abundance in the spring season, also they are having least variety and abundance in the autumn season and maximum density belongs to Simulidae and Chironomidae. The result was indicated that output great Gahar station (fourth station) had the highest density and diversity to another stations. The Biological index of Gahar Lake that obtained 1.75, was achieved by using estimates of water quality based on HFBI, lake water quality was excellent and without apparent organic pollution.