In this study, wood plastic composites were prepared with wood flour (WF) and polypropylene matrix (PP). Injection molds were used to fabricate the test specimens of wood plastic composites. The test specimens contain different percentage of wood flour (10%wt , 20%wt , 30%wt ,40%wt ) . Scanning electron microscopy (SEM) was used to contemplate the distribution of WF in pp matrix. The effect of WF on the free volume properties of PP matrix was investigated using Positron annihilation lifetime spectroscopy (PLAS). The S parameter was estimated for all specimens as a function of the weight percentage of WF. The PLAS results indicated that the mechanical properties depend on free volume. Endeavors were made to correlate the available free volume with mechanical properties which leads to general conviction that a positive correction between the free volume and both hardness and tensile strength.