In this paper, highly regioselective enzymatic acylations of 1-beta-D-arabinofuranosylcytosine (ara-C) with vinyl stearate (VS) in binary organic solvents were explored for the preparation of 5'-O-stearate of ara-C with potential antitumor activity. Twelve kinds of hydrolases were tested for the regioselective acylation reaction and the immobilized Candida antarctica lipase B (Novozym 435) showed the highest regioselectivity (>99.9%) towards the 5'-OH of ara-C. A comparative study showed that the lipase had much higher catalytic activity in the binary mixture of hexane and pyridine than in other tested co-solvent systems. To better understand lipase-mediated acylation conducted in the best binary organic solvent system, the effects of hydrophobic solvent content, molar ratio of VS to ara-C, initial water activity, and reaction temperature on the acylation reaction were studied. It was found that the most suitable hexane content, VS-ara-C molar ratio, initial water activity, and reaction temperature were shown to be 25% (v/v), 20:1, 0.07, and 50 degrees C, respectively. Under these reaction conditions, the initial reaction rate, the maximum substrate conversion, and regioselectivity were as high as 86.0 mmol.L(-1)h(-1), 96.6%, and >99.9%, respectively. The product of Novozym 435-catalyzed acylation was characterized by Carbon-13((13)C) NMR and confirmed to be 5'-O-stearate of ara-C.