Several thermal modification methods are developed, which all involve subjecting wood to high temperatures ranging from 160 to 260 C for several hours in low oxygen environment (Esteves and Pereira, 2009). The methods differ between each other in one or more following aspects: treatment environment (steam, oil, nitrogen or vacuum), reactor system (closed or open) and number of process stages. The most widely used thermal modification (Thermowood®) is three stage treatment in an open reactor system in steam environment (Hill, 2006; Sandberg and Kutnar, 2016, Teng et al. 2018). WTT has developed a similar process, where the only difference is that a closed reactor system is used. Such difference significantly change the technological parameters: lower process temperature, higher pressure and shorter modification time. Besides, in the closed reactor system all of the thermal degradation products are left in the reactor (Militz & Altgen, 2014). Previous studies have concluded that TM wood particles are suitable for production of wood plastic composites (WPC) with improved properties (Segerholm, 2012; Kuka et al., 2016). However, the influence of different thermal modification methods is less studied. The main objective of the present study is to compare and determine how wood residues from open and closed thermal modification processes influence WPC properties.