Abstract:The growth inhibition effect of isothiazolone BIT (1, 2-benzo-isothiazolin-3-ketone) and its analogue X (N-propionyl-1, 2-benzo-isothiazolone ketone-3-ketone) with different concentrations on Phaeodactylum tricornutum were studied. The results of algal cells growth, specific growth rate, algal cells density ratio indicated that the growth of Phaeodactylum tricornutum could be inhibited at the initial growth phase after the algal cells were treated with high concentrations BIT and X. However the treatment groups with low concentrations BIT and X were not obvious compared with the control. The inhibitory effect of BIT was stronger than X, but the effect could decrease with the treatment time prolonged, the algal cells will proliferate rapidly. The EC50values of BIT and X were 1.95mg/L (R2 = 0.988, P = 0.0013) and 3.26mg/L (R2 = 0.908, P= 0.0279) respectively according to the Logistic curve fitting equation. It further showed that BIT had the better inhibitory effect than its analogue X.