The acylation of ethane-1,2-diol catalyzed by lipase (Novozym 435) with ethyl acetate in 1-butyl-3-methyl imidazolium hexaflorophosphate [BMIM][PF6] and 1-butyl-3-methyl imidazolium tetrafloroborate [BMIM][BF4] was studied. It was demonstrated that the activity of the enzyme in [BMIM][BF4] was very low for this reaction, while it was very active in [BMIM][PF6]. Both the equilibrium conversion and the selectivity to ethane-1,2-diol monoacetate (EDMA) in [BMIM][PF6] could be considerably higher than that at solvent-free condition. The IL and enzyme have synergetic effect to prevent thermal deactivation of the enzyme. Addition of small amount of water in [BMIM][PF6] could enhance the conversion and selectivity to EDMA.