In this study, biodiesel is produced from waste frying oil and raw soybean oil by using a transesterification method. The effects of waste frying and raw soybean methyl esters on engine performance and NOx emissions are investigated. By using waste frying and raw soybean methyl esters, a decrease in engine torque and power and an increase in specific fuel consumption due to low heating values of methyl esters are observed. Since the oxygen content of methyl esters affects NOx development, an increase in Nay emissions is also observed.