One of the most important parameters for monitoring, designing and optimizing the algae-based wastewater treatment system is the biomass concentration. The determination method of microalgal biomass concentration in the culture medium by Volatile Suspended Solids (VSS), as a conventional method, was found inconvenient because it takes long time to obtain the results. In this study, the performance of five feasible methods for microalgal biomass determinations was evaluated and compared. The coefficient of determination (R²) of Optical Density (OD), Total Suspended Solids (TSS), turbidity and wet weight was 0.9903, 0.9698, 0.975, 0.9794 and 0.9922, 0.9686, 0.9649, 0.9694 for C. vulgaris and Phormidium sp., respectively. After comparing with the VSS method during wastewater treatment process, OD is recommended as a reliable method for biomass determinations for green (C. vulgaris) and blue-green (Phormidium sp.) algae.