In microcellular foaming process of wood/plastic composites, the alterations in melt viscosity induced by the addition of wood-flour into the plastic matrix play an important role during the growth and stabilization of nucleated cells. This study examined the effects of wood-flour and low molecular weight rheology modifier addition levels on the melt flow index, shear viscosity, and extensional viscosity of PLA/wood-flour composites. From the results, an effective strategy is proposed to successfully and continuously produce microcellular foamed PLA/wood-flour composites with cellular structures similar to those achieved in neat PLA foams. It consists of matching the composites' melt index value (or melt viscosity) to that of neat PLA by using a rheology modifier. Lowering the melt index of the composites from the matching value resulted in unprocessable foaming conditions owing to the increased melt viscosity, whereas cell structures slightly deteriorated by increasing the melt index of the composite above its optimum value.