In this study, the effect of light wavelength on growth rate and lipid production of Synechocystis was investigated. Continuous cultivation system was used to have uniform cell density and avoid self-shading in order to obtain more precise results. Based on previous studies, red light is more efficient than other colors in the visible spectrum for cultivation of Synechocystis; however, the optimum wavelength in red light spectrum remains still unknown. In order to determine the most efficient wavelength of red light, five different wavelengths including 600, 635, 660, 670, and 730 nm were used for growing Synechocystis in a chemostat setup. The results revealed that 635 nm was the most efficient wavelength for cultivation of Synechocystis in terms of both biomass production yield and growth rate. These findings can be attributed to the existence of phycocyanin, the principal light-harvesting supercomplex in Synechocystis, which absorbs maximally at around 620 nm. The results also indicated that cell size and fatty acid profile of Synechocystis were almost the same for different light wavelengths; however, the maximum light was absorbed at 635 nm.