The effects of irradiance on the growth of toxic dinoflagellates Alexandrium tamarense (Masan Bay strain) and Alexandrium catenella (Jinhae Bay strain) were investigated in the laboratory. At $15^{circ}C$ and 30 psu for A. tamarense and $25^{circ}C$ and 30 psu for A. catenella, the irradiance-growth curve showed the maximum growth rate (${mu}_{max}$) of 0.31 $day^{-1}$ with half-saturation photon flux density (PFD) ($K_I$) of 44.53 ${mu}molm^{-2}s^{-1}$, and a compensation PFD ($I_c$) was 20.67 ${mu}molm^{-2}s^{-1}$ for A. tamarense, and ${mu}_{max}$ of 0.38 $day^{-1}$ with $K_I$ of 59.53 ${mu}molm^{-2}s^{-1}$, and $I_c$ was 40.80 ${mu}molm^{-2}s^{-1}$ for A. catenella. The $I_c$ equated to a depth of 8~9 m from March to June for A. tamarense and 6~7 m from March to June for A. catenella. These responses suggested that irradiance at the depth near the middle layer in Masan Bay would provide favorable conditions for two species.