Atrazine is a selective herbicide used for pre-emergence control of broad-leaved weeds. Its frequent presence in ground and surface water has caused public concerns in regard to environmental pollution and human health. A study was conducted to compare the effect of various organic amendments differing in C/N ratio and complexity and also to evaluate the influence of supplemental inorganic N on biological and chemical degradation of atrazine in sterile and active soils. A silty loam soil was taken from the surface 25 cm of an experimental field located on soil and Water Research Centre of Mashhad, Iran. Initial concentration of atrazine was chosen to be 100mg.kg-1 soil. Contaminated soil samples were mixed with vermicompost, manure, glucose, starch and sawdust at a rate of 5% by weight and 250 mg inorganic N as NH4NO3. The samples of the sterile soil treatments were treated with 2500 mg.kg-1 of HgfCl2 to kill microorganisms. Residual atrazine concentration in soil was determined by HPLC at the end of 20,40 and 60 days incubation period. The experiment was performed as a 2*6*2 factorial completely randomized design with 3 replications. Experimental treatments were consisted of 2 levels of soil (active and sterile soils), 6 levels of organic amendments (non-amended soil, vermicompost, manure, glucose, starch, sawdust) and N in two levels (0 and 250 mg). Atrazine degradation in soil seems to have biotic as well as aboiotic origin. Results showed that microbial degradation was the main process responsible for atrazine behavior in soil. However, manure significantly increased the rate of atrazine biodegradation, addition of other organic amendments suppressed this herbicide degradation in active soil compared to non-amended soil. Treatments receiving supplemental inorganic N showed considerably lower rate of atrazine mineralization than corresponding treatments without N. Probably the competition between atrazine-degrading microorganisms and those heterotrophic microflora for space and nutrients may be considered an important factor for decreasing atrazine degradation. No relationship was found between the C/N ratio of the soil and atrazine mineralization. Furthermore, chemical degradation of atrazine in sterile soil was not affected by either organic amendments or inorganic N during 60 days of incubation.