Seasonal growth and maturation process of Caulerpa okamurae were investigated in natural populations and in culture. Monthly sampling was carried out by SCUBA diving at Baekunpo, Busan, southeastern coast of Korea from November 1999 to October 2001. Growth of erect branches depended mainly on the water temperature in the natural habitat. Maximum length of erect branches was 14.0 $pm$ 1.4 cm in June 2001 when the water temperature was 19.7$^{circ}C$ and minimum was 2.8$pm$0.2 cm in April when the water temperature was 14.7$^{circ}C$. Fresh weight of erect branches was 3.9 $pm$ 0.5 g in June 2001 and 0.2 $pm$ 0.04 g in September 2000. Biomass of the population was maximum of 922.5 g dw${cdot}m^{-2}$ in July and minimum of 125.6 g dw${cdot}m^{-2}$ in April. Gametangial network was observed on the ramuli when the water temperature was over 19$^{circ}C$ in August 2000 and June 2001. In the culture regime of 4 temperatures (15, 20, 25 and 30$^{circ}C$) and 3 irradiances (20, 50 and 80 $mu$mol${cdot}m^{-2}{cdot}s^{-1}$) combination, the maturation of excised erect branches was mainly affected by temperature. Maturation was induced under all irradiance conditions at 20 and 25$^{circ}C$; under 80 $mu$mol${cdot}m^{-2}{cdot}s^{-1}$ at 15$^{circ}C$; and under 20 $mu$mol${cdot}m^{-2}{cdot}s^{-1}$ at 30$^{circ}C$. The highest rate of maturation was 64% under 20 $mu$mol${cdot}m^{-2}{cdot}s^{-1}$ at 25$^{circ}C$. These results suggested that developmental initiations of C. okamurae occurred at higher than 13$^{circ}C$ and maturation took about 270 degree-days.