Abstract: By using natural fiber—bacterial cellulose (BC) as the reinforcement and unsaturated polyester resin (UPR) as the matrix，natural fiber BC/UPR composites were prepared by  the RTM process. The mechanical properties，moisture absorption and degradation of the composites were studied. The results show that  chemical bonds are formed at the interface of the BC/UPR composites through surface treatment of BC，which enhances the mechanical properties of the composites. With the increasing fiber volume fraction，the BC/UPR composites get an increase in tensile strength as well as a decrease in elongation at break. The best tensile strength of 152.9 MPa can be obtained for the composites with a fiber volume fraction of 20%. The processes of moisture absorption of the BC/UPR composites，which lower the mechanical properties，can be described by Fick’s Law. UV radiation tests were carried out on the BC/UPR composites. XPS results prove that an oxidative degradation takes place and the contents of oxygenic functional groups increase when the materials are exposed to ultraviolet radiation，which induces photodegradation in the BC/UPR composites.