This study was conducted to clarify the mold resistance and water absorption of wood plastic composites (WPC). Wood/HDPE and bamboo/HDPE with different fiber content were used as test material. Mold fungi resistance (with or without biocide zinc borate (ZB)), scanning electron microscopy (SEM) analysis and water-absorption rate were investigated after 28 days incubation with Aspergillus niger, Trihoderma viride, Penicillium funiculosum and Aureobasidium pullulans. Mold resistance varied with fiber content and the incorporation of zinc borate. Composites contained higher fiber content was more susceptible to mold fungi. The addition of ZB remarkably improved the mold resistance of WPC. SEM analysis showed distinct evidence of mold growth and colonization on WPC specimen. Water absorption of WPC was jointly affected by wood or bamboo content, mold attack and temperature. Water absorption progressed quickly with higher fiber content and temperature and the mold growth also seemed to promote the water absorption of WPC.