Abstract: The crystallite size of the three different carbon fibers (K223HE, HTA40 and T700SC) was characterized by Raman spectroscopy. The transverse elastic modulus and hardness of the carbon fibers were measured by nano indentation technique, and the data scatter was analyzed by two-parameter Weibull function. The results show that the crystallite length of the mesophase pitch based carbon fiber K223HE is the longest, about 39.43 nm±2.63 nm, and that of HTA40 and T700SC PAN based carbon fiber is almost the same, about 4.63 nm±0.09 nm and 4.89 nm±0.06 nm, respectively. In the nano indentation load-depth curve, the residual deformation of K223HE is the biggest (75.34 nm±17.07 nm), indicating the lowest recovery ratio of indentation work (65.89%). The characteristic transverse elastic modulus of the three carbon fibers is 19.52 GPa, 11.99 GPa and 17.92 GPa. The Weibull modulus of the transverse elastic modulus of the three different carbon fibers (HTA40, K223HE and T700SC) is 25.26, 6.85 and 8.07, which displays the consistence in the properties of HTA40 is the best. The differences in the nano indentation behavior of the three carbon fibers are attributed to the differences in the integrity and preferred orientation of the crystallite of carbon fibers.