Synechocystis sp. PCC 6803 was grown in a 2.51 enclosed photobioreactor on medium with or without glucose. The incident light intensities ranged from 1.5 klux to 7 klux. The highest average specific growth rates of mixotrophic culture and photoautotrophic culture were, respectively, 1.3 h(-1) at a light intensity of 7 klux on 3.2 g l(-1) glucose and 0.3 h(-1) at both light intensities of 5 klux and 7 klux. The highest cell density 2.5 g l(-1) was obtained at both of light intensities 5 klux and 7 klux on 3.2 g glucose l(-1). Glucose consumption decreased with decreasing light intensity. The energy yields of mixotrophic cultures were 4 to 6 times higher than that of photoautotrophic cultures. Light favored mixotrophic growth of Synechocystis sp. PCC 6803, especially at higher light intensities (5-7 klux).