The transesterification of waste oil to biodiesel in a solvent free system catalyzed by an immobilized lipase Novozym 435 was explored. The byproduct glycerol was found to be adsorbed on the support and the stability of the immobilized lipase was enhanced by rinsing the enzyme with acetone. The optimal reaction conditions such as molar ratio of methanol to oil, enzyme amount, temperature and shaking rate were 1, 6.6 U/g , 35~40 ℃ and 150 r/min , respectively. Water addition was not favorable for the...