[en] We present comparisons between quantum and classical calculations for above threshold ionization (ATI) of an Argon atom interacting with a strong laser pulse. The time-dependent complete-active-space self-consistent field (TD-CASSCF) method and the classical trajectory Monte-Carlo (CTMC) method are used for the cases of quantum and classical calculations, respectively. Both methods successfully reproduce the main structures of ATI of an Ar atom. In detail structures of angular resolved photoelectron energy spectra and angular distribution of photoelectron yields, the quantum and classical calculations show differences. We found that the difference is remarkable in the directions parallel with and perpendicular to the laser polarization