Abstract: The effects of fiber aspect ratio on the dynamic mechanical properties for both the glass/epoxy and carbon/epoxy composites have been investigated. The results show that the mechanical internal friction of materials reduce and the modulus enhances with fiber aepoct ratio.After a certain aspect ratio both the internal frictionwand modulus begin to levol off and to approach the values of continuous f fiber composite.Therefore,the f fiber utilization cfficicncy is fully played.The internal friction of carbon fiber composite is higher than that of glass one.The higher the test temperature and the lower the matrix modulus,the fiber efficiency will be lower.Hence,the aspect ratio for the same of fiber eff iciency will be higher.The phenomenon mentioned above cold be acccunted by that the stressces in the ffibers and at the fiber/resin interface along the length of the fibers would change with aspect ratio under the external loading or the thermal shrinkage stresses are applied.