Background & objectives: One of the most important environmental problems is soils contamination by heavy metals in industrial areas and agricultural lands. Use of earthworms in soil bioremediation causes decrease in the pollutants concentration through bioaccumulation of the contaminants in earthworms.

Methods: This experimental study was carried out on the soils contaminated with chromium and cadmium at concentrations of 0.04 and 0.08 mg/g along with the control samples. Organic matters (Manure) at the concentrations of 5 and 9% t were added. Chromium and cadmium concentration in soil and the worms were measured at two time periods of 21 and 42 days by ICP spectrometry. Two-way variance was used for statistical analysis and p<0.05 was considered as significant level.

Results: According to the results of this study, the removal efficiency decreased by 5% with increasing the organic material of the soil contaminated with chromium from 5% to 9% at the concentration of 0.06 mg/g. Bioremediation efficiency decreased by 20% at chromium concentration of 0.1 mg/g implying that the earthworms have probably more tendency to consume organic material rather than the soil contaminated with metals.

Conclusion: Considering increased mortality of worms at chromium concentration of 0.08 mg/g, this method is not recommended for soil bioremediation. In the case of cadmium further study is required. One can say that the organic material had no influence in bioremediation of the soil.