Factors influencing the synthesis of mono- and diester of polyethylene glycol400 (PEG400) oleate with immobilized lipase from Candida sp.1619 as biocatalyst were investigated. Mono- and diester were formed with different molar ratios of the substrates after reacting for 6 hours. The amount of monoester to diester formed was in the range of 3.5:1 to 4.1:1 when the molar ratio of acid to PEG400 was 0.25:1 to 2:1. Almost equal amounts of mono- and diesters were produced when the molar ratio of acid to PEG400 was 3:1 to 8:1. When the equilibrium of reaction was reached (22 hours), only diester was found in the reaction mixture with different molar ratio of substrates. The amount of monoester to diester was 1:3.2 in the reaction system containing hexane, even the molar ratio of acid to PEG400 was 2:1.