Ultraviolet irradiation could be one of the alternatives to conventional treatments against algal bloom. In this study, the efficacy of UV treatment was investigated against the growth of indigenous Microcystis collected from the Terauchi reservoir, which was suffering from algal bloom on the day of sampling. Surface water from the reservoir was subjected to lab-scale UV exposure using a low-pressure UV (LP UV) lamp or a medium-pressure UV (MP UV) lamp. UV-irradiated samples were compared with non irradiated control samples to determine the growth inhibitory effect of UV irradiation. Several exposure conditions were adopted by varying UV fluence and UV fluence rate to investigate their effects on algal growth. The growth inhibition observed after MP UV irradiation was about 3-fold higher than that after LP UV irradiation, up to 50 [mJ · cm^-2]. In the case of MP UV irradiation, growth inhibition was almost constant regardless of UV fluence rate at constant UV fluence. A 1.35 [mW · cm^-2] UV fluence rate resulted in a growth inhibitory effect 4.6-fold higher than that of a 0.37 [mW · cm^-2] UV fluence rate at constant UV fluence in the case of LP UV irradiation. The results of this study indicate that UV irradiation is effective in inhibiting the growth of indigenous Microcystis.