Relative lower strength and poor creep resistance of wood-plastic composites (WPC) restrain their wider applications in building and automotive. A novel wood-polymer composite was prepared using Kraft fiber and unsaturated polyester (UPE), which had much higher strength and better creep resistances than that of traditional thermoplastic WPC. Effects of initiator on the mechanical properties and creep resistance of this novel composite were investigated by tensile evaluation, DMA, SEM and short-term creep test. Test results indicated that initiator level had important effects on mechanical properties and viscoelastic behaviors because of various crosslinking densities of UPE matrix and interface adhesions between Kraft fiber and UPE resin under various initiator levels. With initiator level increased from 0.3% to 1%, the tensile strength and interface adhesion increased at the beginning and then decreased, while the instantaneous strain and maximum strain in the creep test decreased gradually. As for hot molding at 125°C, initiator level shall be less more than 1% and be preferable to 0.5%-0.7%.