From Fig. 2, pH 5 was identified to provide the most suitable environment for yeast and enzyme to work efficiently for bioethanol production. Bioethanol yield reaches its optimum value of about 93.76% at the fermentation time of about 180 min. Compared to others curve line, the sharp increase demonstrated that enzyme converts most of microalgae starch into glucose, which is subsequently utilized by the active yeast to produce bioethanol. Biothanol yield was found to decrease significantly at this pH after the peak most probably due to starvation since the residual sugar content was almost negligible in the medium. This phenomena was reported by Harun et . and Tan et al.