The elastic modulus and hardness of interphase region for both Kevlar-49/epoxy and F-12/epoxy unidirectional composites were analyzed using quasistatic nanoindentation method. The results show that the height difference of aramid fiber and epoxy resin in longitudinal section of unidirectional composites is low. Therefore, the experimental error induced by indenter shape can be reduced, so that the mechanical properties of interphase region can be analyzed more accurately. The elastic modulus and hardness of interphase region are distinctly different from those of aramid fiber and epoxy resin. Based on the results of mechanical properties for fiber, epoxy and interphase obtained from nanoindentation, it is assumed that the width of property transition region is less than 1μm.