In previous reports, the authors isolated the algicidal marine bacterium, Alteromonas sp. SR-14 and demonstrated its growth inhibition of diatom, Chaetoceros calcitrans (C. calcitrans). In this paper, we studied the effects of cell free culture filtrate of Alteromonas sp. SR-14 on the growth of C. calcitrans, and the characteristics of the algal growth inhibition substance. The culture filtrate of Alteromonas sp. SR-14 grown in peptone broth showed growth inhibition activity against C. calcitrans. The reasonable culture conditions of the bacterium for producing of algal growth inhibition substances were $15~20^{circ}$ in temperature, 7.0-9.0 in pH and $23~30{ extperthousand}$ in salinity, respectively. The algal growth inhibition activity of culture filtrate was increased from stationary phase in growth curve of Alteromonas sp. SR-14. The molecular weights of algal growth inhibition substances produced by Alteromonas sp. SR-14 were ranged about from 3 KDa to 12 KDa. Among the substances, less than 10 KDa fraction were stable by heating at $100^{circ}$ for 10 minutes, while more than 10 KDa fraction were heat labile. According to the experimental results, the algal growth inhibition substance produced by the bacterium was not a single compound.