We present a mode field analysis of the second‐harmonic electromagnetic wave that radiates from a nonlinear core bounded by a dielectric cladding. With this analysis the ultimate performance of the organic crystal‐cored single‐mode optical fiber waveguide as a guided‐wave frequency doubler is evaluated through the solution of nonlinear parametric equations derived from Maxwell’s equations under some assumptions. As a phase‐matching scheme we consider a Cerenkov approach because of advantages in actual device applications, in which the phase matching is achievable between the fundamental guided LP01mode and the second‐harmonic radiation (leaky) mode. Calculated results for organic cores made of benzil, 4‐(N, N‐dimethyl‐amino)‐3‐acetamidonitrobenzen, 2‐methyl‐4‐nitroaniline, and 4’‐nitrobenzilidene‐3‐acetoamino‐4‐metxianiline provide useful data for designing an efficient fiber‐optic wavelength converter utilizing nonlinear parametric processes. A detailed comparison is made between results for infinite and finite cladding thicknesses.