Members of Sphingomonas genus played a key role in the biodegradation of polycyclic aromatic hydrocarbons (PAHs). However, little was available in the literature about diversity and population structure of genus Sphingomonas in petroleum-contaminated soils, and relationships between Sphingomonas and PAHs based on direct detection Sphingomonas strains with Sphingomonas genus-specific primers. In this paper, total soil DNA from the soil in Shenfu irrigation zone were amplified with newly designed Sphingomonas genus-specific primer set 732f-982r and then these amplified DNA fragments were separated by DGGE for analysis of genetic diversity of Sphingomonas in the soil of the irrigation zone. Results indicate that the newly developed primer set 732f-982r could be used for specific detection of strains belonging to the genus Sphingomonas. When PAHs contamination of the soil is at a low level, i.e. less than 5 mg kg^-1, the diversity of Sphingomonas increased with rising PAHs concentration; but when the initial concentration of PAHs in lab incubation experiment is relatively high, the diversity of Sphingomonas decreased. Different concentrations of total petroleum hydrocarbons (TPH) lead to difference in Sphingomonas population, and soils the same in TPH contamination level are similar in Sphingomonas population.