Abstract: In order to understand the interspecies competition of Chattonella marina with other harmful algal bloom species, and to provide useful information for the control of C. marina bloom, the growth of C. marina and competition between C. marina (Cm) and two other harmful algal bloom (HAB) species, Skeletonema costatum (Sc) and Scrippsiella trochoidea (St), were studied, Experiments were conducted at three initial inoculation proportions according to their cell volumes, and four Si concentrations. The competition parameters were used to compare interspecific competition between the three algae, and the concept of volume competition parameter was firstly introduced. The carrying capacity (K) of C. marina showed no obvious changes in five different initial cell densities ranged from 100 cells/mL to 3600 cells/mL, however, the maximum cell density and the stable stage postponed with the decreasing of initial cell density. S. costatum had the ability to keep steady growth even in Si free culture, and held the predominance of cell number in the competition of C. marina. Meanwhile, the increase of Si level benefited the competition of S. costatum to C. marina. The initial cell density and inoculation proportion exhibited significant effects on interspecific competition of the three species. Although S. costatum grew faster than C. marina in the same initial cell density (600 cells/mL), the volume competition parameters between them (α' and β') were comparable. S. costatum won in the culture when its inoculation cell number overpassed that of C. marina (Cm:Sc=1:6, the same cell volume). C. marina predominated in the culture only when its cell number greatly preponderated over S. costatum (Cm:Sc=6:1). S. trochoidea produced strong inhibition effects on the growth of C. marina under all treatments of the experiment (Cm:St=1:1, 1:3, 3:1), and its volume competition parameters to C. marina (α') were 27—100 times higher than the opposition (β'). However, no out-competionts were observed in all treatments of this study, which implied that all of the three species had the possibility to dominate in the interspecific competition. From the results of this study, it could be concluded that background cell number and proportion of HAB species in the water column may play important roles in phytoplankton competition, and the background cell number would be a useful parameter for the forecast of C. marina bloom.