In this experimental study, biodiesel production from waste oils (trap grease), which were obtained from an oil separator, was carried out by using sulfuric acid as the catalyst and esterification process. The main variables involved in the esterification process, including methanol/waste cooking oils ratio, amount of acid catalyst, reaction time, and reaction temperature, were analyzed. Because the waste cooking oils contain high free fatty acid above 65%, the esterification process has been preferred. Methanol was used as alcohol in this process because of its low cost, and sulfuric acid was preferred as the catalyst because it gives very high yields in alkyl esters. Experiments have been performed to determine optimum conditions for this esterification process in different molar ratios, catalyst amounts, reaction times, and reaction temperatures. The optimum experimental conditions, which were obtained from the esterification process, were methanol/waste cooking oils ratio 9/1, with 9 wt% sulfuric acid catalyst, reaction time 120 min, and reaction temperature 60 degrees C. The yield of biodiesel was 93.98% at optimum esterification process. Biodiesel and its blends with diesel were characterized for their physical properties referring to a substitute for diesel fuel. The results show that blends with a percentage of the biodiesel below 40 vol% had their physical properties within EN590 standard, which indicated that these could be used in engines without a major modification.