Buffer fat was interesterified in a cosurfactant-free microemulsion system containing nonionic and ionic surfactants, using commercial lipase obtained from Rhizopus niveus, at different concentrations of surfactant mixtures and hydrophilic-lipophilic balance (HLB) values. The results indicated that the interesterification yield (IY) of lipase-catalyzed interesterified butter fat reached its maximum in the microemulsion system prepared with the surfactant mixture of HLB value of 9, followed by that of HLB value of 10. In addition, increasing concentrations of surfactant mixtures, from 3 to 6 mM, resulted by an increase in the IY. The interesterification of butter fat in the microemulsion prepared with 3 mM of surfactant mixture of HLB value of 10 showed a minimum hydrolytic activity. The results showed that the interesterified selected triacylglycerol molecules were enriched with the hypocholesterolemic fatty acid C18:1, originally located on sn-1,3 positions, on their sn-2 positions; this fatty acid was favorably interchanged with the hypercholesterolemic fatty acid C16:0, originally located on sn-2 position. The results also indicated that the use of 6 mM of surfactant mixtures increased the acyl exchange reaction toward the long-chain saturated fatty acid C16:0 an the sn-2 position of triacylglycerol molecules.