There is distinct deviation between the intrinsic and measurement value of Young's modulus in uniaxial compression tests. The key influence factor has always been owned to the friction between compression platens and specimen. However, we found this is not true by Finite element analysis (FEA) and experiments. FEA results show that the friction only makes a major influence on the plastic behavior, resulting in a barreling deformation, rather than on the elastic deformation behavior of specimen. In fact, the oblique contact between compression plate and specimen will severely influence the homogeneity of the stress field in specimen, and result in the deviation of the measured modulus value. This study can well interpret the relevant provision in ASTM E9-09 that only slim specimens (with aspect ratio >= 8) are suitable for the compression modulus tests.