Abstract: The objectives of this study were to investigate the abundance, biomass and practical distribution of earthworms in ecosystem and tried to identify the factors affecting earthworm populations during different environmental conditions. Density and biomass of earthworms were studied in ecosystem unit`s khanikan forests (North of Iran) in July 2006. Eighteen soil profiles (50x50 cm) to the depth of 30 cm were digged and soil samples were taken from organic horizon (litter layer) and mineral layers (0-10, 10-20 and 20-30 cm). Earthworms were collected by hand sorting method, then oven-dried at 60°C and weighed. Comparison number and biomass of earthworms in various layers of soil have showed that most number was in third layer (63.63%) and the least number was in second layer (13.63%). Also, biomass of earthworms in third layer was the most (79.39%) and was the least in second layer (7.57%). Results of this research indicated that correlation between number and biomass of earthworms with C/N, biomass of earthworms with carbon of soil and number of earthworms with theirs biomass were significant. Correlation between number and biomass of earthworms with the other soil properties investigated was no significant.