Abstract: Effects of biodegradation on the attenuation factor (a_i-s ) and screening level of benzene in the soil vapor were studied by using the one dimensional model, Bio-vapor for sandy soil, in particular on critical influential factors of biodegradation were simulated and discussed, including source intensity (c_s), vertical separation distance between building foundation and source (LT), depth of aerobic zone in soil (L_a), biodegradation rate (k_w ). The results indicated that contribution from biodegradation of benzene to a_i-s was negligible, when c_s was higher than 5×10⁵ mg/m³. When c_s was less than 1×10⁴ mg/m³, a_i-s was 1-2orders of magnitude reduction when the aerobic biodegradation process was included compared to the non-biodegradation case, but the reduction is insensitive to the change of c_s and LT. When c_s falls in between, a_i-s decreases by 2orders of magnitude as LT increases by 1order of magnitude, and LT is a sensitive parameter. L_a is a critical factor in determining the biodegradation effect, and bio-attenuation decreases by 2orders of magnitude as L_a just increases from 0.50to 1.50m. Compared with the field measured results for L_a, which were always deeper than 1.50m, the modeled results for the sandy soil was 0.63m, indicating that Bio-vapor is conservative in predicting the depth of aerobic zone. Therefore determination of L_a through measuring the soil vapor profile in practical project is recommended. Bioattenuation was more sensitive to k_w for light contaminated sites, and a_i-s decreases by 2orders of magnitude as k_w increases from 0.033to 2.000h^-1 when c_s is no more than 5×10⁴ mg/m³. The vapor screening value for benzene is 1-2orders of magnitude lower when biodegradation is considered for the same conceptual model scenario with no biodegradation.