Abstract: Cassava starch was used as raw material,rosin acid as esterifying agent,chloroform as solvent,and liquid lipase as catalyst to synthesize rosin starch ester(RAS)directly on the surface of native cassava starch in two-phase system. The structure and physical and chemical properties of samples were analyzed by fourier transform infrared spectroscopy(FT-IR),scanning electron microscopy(SEM),X-ray diffraction(XRD),thermo gravimetric analyze(TGA),contact angle,emulsifying ability and emulsion stability. The results showed that the characteristic carbonyl peak of RAS near 1728 cm^-1 in FT-IR spectrogram and the esterified spots on the surface of RAS in SEM spectrogram indicated that the rosin acid group successfully attached to the surface of the cassava starch. XRD and TGA results showed that the crystallinity of tapioca starch decreased after modification with rosin acid. Compared with the native cassava starch,the contact angle of RAS(DS=0.0502)increased 63.40%. The emulsifying ability and emulsion stability increased from 1.53% and 1.50% of the native cassava starch to 31.91% and 23.08%,respectively. And as the DS increased,the hydrophobic properties and emulsifying properties of RAS also increased.