Biosolids contain nutrients, organic matters and micro-organisms that can provide soil benefits. In this study toluene and naphthalene-contaminated soils were used to investigate the effect of applying biosolids on the enhancement of the biodegradation rate. The oxygen uptake rate (OUR) was determined with a respirometer and was used to calculate the oxygen uptake percentage of substrate in biosolids (alpha value) using a two-phase respirogram. Experimental result showed that the application of biosolids had positive effect on the enhancement of the biodegradation rate of toluene and naphthalene in the contaminated soils. The biodegradation rates of toluene and naphthalene were 15% and 20% in soils without applying biosolids, respectively. With the biosolids, its biodegradation rate for the two contaminants was about 4-fold higher in relative to control and the maximum value occurred in a soil to biosolids ratio as 1: 0.5. The alpha value for toluene and naphthalene was in the levels of 10-20% which revealed that the biosolids used in this study was mainly composed by microorganism.