Rubber seed oil (RSO) is extracted from rubber seed kernel by using screw press and used as a feed stock for biodiesel synthesis via acid treatment. The yield of RSO from rubber seed kernel is about 36 % and free fatty acids (FFA) content of this oil is 9.288%. Due to high acid value of RSO, acid treatment of RSO has been carried out to reduce FFA and purification process is followed to synthesize biodiesel. Acid treatments (esterifications) of RSO are carried out to study the influence of: including molar ratios of methanol to RSO, quantity of sulfuric acid catalysts, reaction temperatures and reaction times. The best condition of acid treatment is molar ratio of methanol to RSO 6:1, 1% ( vol. %) of sulfuric acid, temperature 50ºC, reaction time of 30 minutes and the yield of esterified rubber seed oil (ERSO) about 95%. The amount of FFA is reduced from 9.288% to less than 2% at the end of the acid treatment. ERSO obtained from the acid treatment is purified by using sodium hydroxide and methanol solution to synthesize biodiesel. The best yield of biodiesel about 89% is obtained when the operating condition of the purification process (neutralization and transesterification) is 9:1 molar ratio of methanol to ERSO, 0.5 wt. % of sodium hydroxide, temperature of 50ºC, reaction time of 30 minutes and stirring rate of 750 rpm. The prepared biodiesel from this study is analyzed and found that its properties meet within the limits of ASTM specifications of biodiesel.