An asphaltene-degrading bacterium S, which can utilize asphaltene as the sole carbon source, was screened and isolated from oil production water. Strain S was showed 99 % probability of Bacillus subtilis Bsn5 by the result of the partial 16SrDNA analysis. The utilization of heavy oil containing abundant asphaltene by strain S was done, and the influence of biodegradation on the physical and chemical properties of heavy oil was analyzed. The results indicated that the surface appearance and structure of heavy oil were changed obviously after biodegradation. The alkane content increased while the asphaltene fractions decreased, compared with these components in heavy oil sample before biodegradation. The asphalt degradation rate is up to 34%, and the viscosity and density of heavy oil was reduced about 34.11% and 4.57% respectively. The physical and chemical properties of heavy oil have been improved by strain S.