Abstract: In recent years, harmful algal blooms (HABs) have caused significant damage to ecosystems and public health around the world,and the bloom occurrences are increasing in frequency and intensity. The dinoflagellate Gymnodinium impudicum is a typical HAB species, isolation and characterization of algicidal bacteria will be beneficial to control its blooms. In this study, isolation of an algicidal bacterium LD-B3 against G. impudicum was performed. 16S rDNA analysis showed that LD-B3 belonged to genus Pseudoalteromonas. After 72 h of LD-B3 bacterium culture addition, the algicidal rate reached up to 81.06% after adding with 2.0% at final concentration. As result, algicidal activity was shown both concentration and time dependent. The results of microscopic observation and measurement of physiological parameters showed that strain LD-B3 could lead to the breakage of Gv. impudicum cell chain and the decrease of Fv/Fm. Moreover, our results indicated that strain LD-B3’s extracellular compounds were involved in algicidal activity, and these substances, probably a protein compound, were sensitive to temperature. Generally, these findings indicated that the strain LD-B3 is a potential source of bacterial to mitigate G. impudicum blooms.