This paper describes the influence of the optical inhomogeneity of a nonlinear crystal on the frequency doubling for transversely homogeneous beams. Using coupled wave equations,the analytic expressions for the longitudinal optical inhomogeneity of a nonlinear crystal and the frequency doubling efficiency, and the approximate formula for determining the optimum crystal length have been found. The influences of longitudinal and transverse optical inhomogeneities on the frequency doubling have been compared.