Sieved mixed wastes can be effectively used as geomaterials after solidification/stabilization treatment of the wastes using steelmaking slag and blast furnace slag. However, there is a concern that wood chips in the sieved mixed wastes will have a significant impact on the mechanical properties of the treated waste. Thus, this study conducted consolidated-drained triaxial compression tests on mixtures of wood chips and composite of a granular type steelmaking slag and a fine powder type blast furnace slag to elucidate the influence of wood chips on the mechanical properties of mixed wastes amended with slags. Fibrous and granular wood chips were used in this study. Wood chips-to-composite slag ratios from 0 to 67 vol.% were used for the tests. The specimens were cured from 0 to 1008 days. It was found that when the wood chip ratio was less than 10 vol.%, the shear strength and rigidity increased until 84 days of curing, but there was no further improvement thereafter, regardless of the wood chips used. In the case of 33 vol.% fibrous wood chips and 67 vol.% granular wood chips, the shear strength and rigidity hardly changed during 0-1008 days.