Engineering plasticsin vehicle generally present significant strain rate dependence of the tensile strength and fracture be-havior.Experimental characterization of the material at high strain rates is crucial for finite-element crash simulation.Specimen size is an issue of importance on dynamic testing for plastic material.According to the recommendation of SAE J2749,specimen sizes of ISO 527 1BA and ASTM D638 Type-V were compared to investigate their effects on the tensile properties of PP(Polypropylene).Three strain rate conditions,0.001-1 s,100-1 s,and 400-1 s,are considered respectively.The frequency of force signal and test system were measured,and the number of reflected waves propogate in the gauge section was calculated.Results suggest that more accurate data can be obtained in the dynamic tests using small specimen than that of large specimen.