In this study, nano-sized WO3 powder was dispersed into Co4Sb1.7Te0.3 thermoelectric matrix by ball milling and hot-press sintering technology. The results of scanning electron microscopy (SEM) showed that the WO3 phase distributed uniformly in the form of inclusions on the matrix. High-resolution transmission electron microscopy (HRTEM) presented that the average size of embedded nano-WO3 was about 50 nm. With the content of WO3 increasing, the electrical conductivity (σ) of Co4Sb11.7Te0.3 decreased and the absolute value of Seebeck coefficient (a) increased. The absolute value of a for 1.5% WO3/Co4Sb11.7Te0.3 composite was 258 µV/K at 650 K. The minimal lattice thermal conductivity (K(L)) of WO3/Co4Sb11.7Te0.3 achieved in the present work was 1.52 W/m K at 650 K. The total thermal conductivity (K) of WO3/Co4Sb11.7Te0.3 composites decreased obviously, which resulted from the phonon scattering by the WO3 inclusions locating on the grain boundaries of Co4Sb11.7Te0.3 matrix. The highest ZTof 0.71 was achieved at 750 K for 1.5%WO3/Co4Sb11.7Te0.3 composite.