Wood fiber/poly(lactic acid) composites with 70 wt% wood fiber were prepared using a super mixing-hot press method. The biodegradability of composite panels made with one unmodified and three types of modified poly(lactic acid) was evaluated by comparing the mechanical strength, molecular weight, thermal properties and microstructures of composite wood fiber/poly(lactic acid) and 100% poly(lactic acid) panels before and after burial in soil. We found that wood fiber/poly(lactic acid) composites degraded more readily than 100% poly(lactic acid) panels during burial. The type of poly(lactic acid) used in wood fiber/poly(lactic acid) composites affected biodegradability. The flexural strength and modulus of wood fiber/poly(lactic acid) panels decreased sharply after burial in soil for 6 months. The molecular weight of poly(lactic acid) in wood fiber/poly(lactic acid) composites decreased more rapidly than 100% poly(lactic acid) panels that had been buried in soil for 6 months. Changes were observed in both the amorphous and crystalline structure of specimens buried in soil. The wood fiber/poly(lactic acid) composites prepared with unmodified poly(lactic acid) have better mechanical properties and degraded more easily during burial than did composites made with modified poly(lactic acid).