We report the observation of second harmonic generation (SHG) from odd-layer MoS2 atomic crystal. In contrast, no SHG is observed for samples with even layer numbers due to the restoration of perfect inversion symmetry. Moreover, the SHG intensity is found to directly reflect the underlying 3-fold rotation symmetry of the crystal, which provides a powerful method for optical imaging of the material crystal structure with sub-micron resolution. © 2013 Optical Society of America.