According to the phase-matched condition of the non-collinear harmonic wave generated effectively in a non-linear crystal, the analytic expressions for the harmonic ring produced by collinear and non-collinear fundamental waves have been obtained. The relations between the size of harmonic ring and the direction of fundamental wave can be deduced from these analytic expressions. The harmonic rings of RDP and LiNbO3 crystals have been observed experimentally. Experimental results are in agreement with this theory.