Research of interaction mechanism between Chlorella vulgaris and two bacterial strains(Z-QD08 and Z-QS01)were conducted under laboratory conditions.Growth rates of bacteria and C.vulgaris were tested under co-culture conditions to evaluate the effects of concentrations of C.vulgaris and bacteria on their interactions.To test whether the availability of inorganic nutrients,vitamins and trace metals affects the interactions between C.vulgaris and bacteria,experiments were performed with or without the culture medium filtrate of C.vulgaris or bacteria.The results showed that the growth of C.vulgaris was promoted at low concentrations of bacteria(5×106cells/ml),and expressed a positive correlation with the bacteria density,whereas opposite trend was observed for treatments with high bacteria density(10×106cells/ml and 20×106cells/ml).The growth rate of bacteria decreased with the increasing concentrations of C.vulgaris.The growth of bacteria Z-QD08 was inhibited by C.vulgaris through interference competition,while the mechanism for interaction between bacteria Z-QS01 and C.vulgaris was resource competition.The influence of cell density on the interaction between microalgae and bacteria was also discussed.These experiments confirm some elements of published theory on interactions between heterotrophic bacteria and microalgae and suggest that heterotrophic bacteria play an important role in the development of blooms in natural waters. Research of interaction mechanism between Chlorella vulgaris and two bacterial strains (Z-QD08 and Z-QS01) were conducted under laboratory conditions. Growth rates of bacteria and C. vulgaris were tested under co-culture conditions to evaluate the effects of concentrations of C. vulgaris and bacteria on their interactions. To test whether the availability of inorganic nutrients, vitamins and trace metals affects the interactions between C. vulgaris and bacteria, experiments were performed with or without the culture medium filtrate of C. vulgaris or bacteria. that the growth of C. vulgaris was promoted at low concentrations of bacteria (5 × 106 cells / ml), and expressed a positive correlation with the bacteria density, whereas opposite trend was observed for treatments with high bacteria density (10 × 106 cells / ml and 20 × 106 cells / ml). The growth rate of bacteria decreased with the increasing concentrations of C. vulgaris. The growth of bacteria Z-QD08 was inhibited by C. vulgaris through in terference competition, while the mechanism for interaction between bacteria Z-QS01 and C. vulgaris was resource competition. The influence of cell density on the interaction between microalgae and bacteria was also discussed. the experiments confirm some elements of published theory on interactions between heterotrophic bacteria and microalgae and suggest that heterotrophic bacteria play an important role in the development of blooms in natural waters.