Abstract: Design of experiments was applied in this study to identify the effects of interaction between environmental conditions on the growth rate of microalgae Chlorella vulgaris. Chlorella vulgaris were cultivated in flat panel photo-bioreactors with 3 L modified Bold’s Basal Medium. A model that can be used to predict the growth rate of Chlorella vulgaris with varied bicarbonate concentration (2.5-7.5 g L-1), nitrate concentration (0.5-1.5 g L-1) and light irradiance (2000 LUX-7000 LUX) has been developed. The results showed that the maximum growth rate was obtained at 5 g L-1 bicarbonate concentration, 1 g L-1 nitrate concentration and light irradiance of 4500 LUX with average value of 0.801 day-1. Besides that, analysis of variance showed that bicarbonate concentration, nitrate concentration and light irradiance individually have significant effect on the growth rate of Chlorella vulgaris. Significant interactions were also identified between bicarbonate and nitrate concentrations, as well as nitrate concentration and light irradiance. It is found that with increasing cell population, higher bicarbonate concentration is required to meet the high carbon source demand in culture medium for photosynthesis of microalgae. On the other hand, the photo-inhibition effect of growth rate due to strong light irradiance could be reduced by increasing nitrate concentration in culture medium.