This study optimized biodiesel production by Monoraphidium sp. in terms of fatty acid methyl ester (FAME) yield by varying four variables (volume of methanol, reaction temperature, reaction time, and ultrasound power) using a Box-Behnken design (BBD). Within the experimental range, the volume of methanol was found to be the most important factor, having a positive influence on the FAME yield. The test variables affected the FAME yield, and the optimal condition varied between alkali- and acid catalyzed transesterification. Both types of transesterification reaction occurred at their optimal catalyst concentrations, 0.5% NaOH and 2% H2SO4, respectively, for both dry and wet microalgal biomass. The FAME yield with the acid-catalyzed reaction was better than that with the alkali-catalyzed reaction. The combined alkali-and-acid-catalyzed transesterification reactions enhanced the FAME yield. The micro algae produced fatty acids, comprising mainly palmitic acid (C16:0), oleic acid (C18:1), linoleic acid (C18:2), and linolenic acid (C18:3), with palmitic acid (C16:0) and linoleic acid (C18:2) being the most abundant. (C) 2017 Elsevier Ltd. All rights reserved.