Degradation of Poly (L-lactide-co-glycolic acid-co-beta-Benzyl-L-aspartate) [P(LLA-BMD)] in vitro was carried out in phosphate buffer solution (PBS) at 37 degrees C for 90 days. Poly (L-lactide-co-glycolide) (PLGA) was used as the control sample. Using gravimetric (weight loss) method, scan electron microscope (SEM), FT-IR, H-1-NMR, Gel permeation chromatography (GPC) and contact angel test as the testing methods, the degradation properties of this copolymer were obtained. The results showed that the copolymer of morpholine-2,5-dione derivative and L-lactide had a lower contact angel (74 degrees) and slower degradation rate than that of PLGA. Under the current experimental conditions the degradation mechanism of P(LLA-BMD) is probably bulk degradation, being the same as PLGA. And the degradation of P(LLA-BMD) was more easily to be under control than PLGA, what's more, the mole composition had hardly changed ( from n(LLA):n(BMD)=83/17 to 85/15 in 90 days). It would be served as a potential candidate material in the delivery of water-soluble drugs.