The maleic acid-castor oil monoester (MACO) was synthesized and was used as monomer to synthesize a new potentially environmentally friendly copolymer of styrene and MACO (poly-St/MACO) by suspension polymerization. Under the appropriate conditions, the poly-St/MACO with yield of 81%, number average molecular weight of 44100?g/mol, and molecular weight distribution of 1.5 could be obtained. The chemical structures of the MACO and resulting copolymer were confirmed by Mass Spectrometry Infrared Spectroscopy and Nuclear Magnetic Resonance Spectroscopy H[1]. The results of thermogravimetric analysis and biodegradation test showed the poly-St/MACO can be used as a new potentially environmentally friendly material with excellent thermal stability.