[Objective] To investigate the effect of crude oil concentrations on bacterial community structures and diversity, and attempt to elucidate the mechanism of this effect. [Methods] Seawater was sampled near a marine drilling platform and was treated with five different crude oil concentrations (0 to 10 g/l) for a week in the laboratory. Then the bacterial communities were detected using terminal restriction fragment length polymorphism (TRFLP) method. [Results] Some new findings were exhibited, such as the bacterial diversity did not simply decrease with the increase of oil concentrations, but decreased at first, then increased and decreased again. Bacterial communities of treatments with 0.1 g/L (M0.1) and 0.5 g/L (M0.5) crude oil, treatments with 2.5 g/L (M2.5) and 10 g/L (M10) crude oil were similar, respectively, and bacterial communities in the oil added groups (M0.1, M0.5, M2.5, M10) were significantly different from the control (M0). Classification results of the dominant terminal restriction fragments (TRFs) in the oil treatments mainly attached to Proteobacteria, Firmicutes and Bacteroidetes. Based on the relative ratios of TRFs in different treatments, the 52 TRFs could be divided into six types (I to VI): low (I)/middle (II)/high (III)/broad (IV)/narrow (V) concentrations of crude oil adapted bacteria and crude oil sensitive bacteria (VI). Furthermore, “Carbon & Energy sources-Toxicities” hypothesis was proposed to explain effects of oil pollution on bacterioplankton. [Conclusion] Impacts of crude oil pollutions on the marine bacterioplankton are closely related to the concentration of oil and the original bacterial communities in the seawater, these bacteria could be classified upon their adaptability to the crude oil. And the “Carbon & Energy sources-Toxicities” hypothesis could explain effects of oil pollution on bacterioplankton very well.