The influence of epoxidized soybean oil (ESO) plasticizer on the mould and algal resistances of wood plastic composites (WPCs) was studied using artificial accelerated tests. The macro- and micro-morphology of the colonization of algae and mould on the surface and fracture morphology of the WPCs samples were observed by digital camera and scanning electron microscopy (SEM). The water absorption and thickness swelling rates of different WPCs specimens with various addition amounts of ESO were also tested. The results indicated that as more of the ESO was added, the mould or algal resistance became weaker, especially on the surfaces of the specimens, which became harshly colonized when the addition amount of ESO was beyond 15 phr. The colonization speed and intensity of the mould were higher than the algae at the same addition level of ESO and the same testing time. The hyphae and spores of mould, but not algae, were found on the inner fracture layers.