Wood plastic composites (WPC) are mainly used as an outdoor material, so durability against fungal decay is one of the factors that should be analyzed and if necessary improved. WPC are susceptible to biodegradation, although these materials have limited water absorption because of the wood fiber encapsulation in polymer matrix. In the study two different water pretreatment methods (short-term and long-term) were used to ensure appropriate water content for fungal growth. Also in the paper thermally modified wood (different regimes) fiber influence on WPC fungal resistance is investigated. The results showed that long-term water pretreated WPC specimens had more suitable conditions for fungal degradation that led to higher weight loss. The results which were related to thermally modified wood fibers showed, that WPC with thermally modified wood fibers had improved resistance against fungi. Thermal modification regimes had an effect on WPC durability as well