A series of biodegradable lactic acid based Polyurethanes modified by castor oil (PLBA-PUs) have been successfully prepared by using a two steps method as follow: Firstly prepolymers with hydroxyl terminated are synthesized by copolymerization of L-lactic acid and 1, 4-butandiol (BD), and then react with castor oil (C.O.) and hexamethylene diisocynate (HDI). The effects of BD/LA monomer ration and C.O./prepolymers mole ratio on the molecular weight and mechanical properties of PLBA-PUs are investigated. The polymers obtained are characterized by gel permeation chromatography, fourier transform infrared spectroscopy, 13C NMR, differential scanning calorimeter (DSC) and tensile testing. Their average molecular weight is over 280,000. They display excellent mechanical properties, such as a tensile strength as high as 31MPa, a tensile modulus as low as 20 MPa, and an elongation at break of 176%. Due to the biocompatibility, these PUs could find applications in biomedical fields, such as soft-tissue engineering.