The domain wall regions in periodically poled MgO-doped LiNbO3 (PPMgLN) crystal are examined by using second harmonic generation (SHG). The results show that the average domain walls that separate the individual domains have a width in the order of about 1 μm, and the walls are neither completely smooth nor uniform along the walls from the imaging of the SHG. It is proposed that the origin of the second harmonic signal in the regions around domain wall is attributed to the non-180o domain walls in the ferroelectric domains.