A marine microalga, Schizochytrium limacinum SR21 has been found to grow fast on crude glycerol- a cheap by product from the current biodiesel industry. Crude glycerol is a mixture of glycerol, soap, methanol, and other impurities. Using crude glycerol from biodiesel production line adopting restaurant used oils as feedstock, we have determined that 20 g/l is the optimal dose. With this glycerol concentration, we have achieved a biomass dry weight as 8.3 g/l in 7 days. During this time frame, glycerol was consumed completely. When crude glycerol was treated to remove soap which is detrimental to cell growth, 35 g/l was identified as the optimum dose. With this glycerol concentration, biomass dry weight as 13.3 g/l was observed in 6 days while glycerol was used up at the same time. Using crude glycerol derived from biodiesel production line started with animal fats, we have obtained similar results. Thus, S. limacinum SR21 is an excellent candidate for converting crude glycerol to biomass. The produced biomass with high lipid content can be utilized either for biodiesel production, as fish meal, or for high-value omega-3 fatty acid production.