Abstract The main goal of this work is to rehabilitate the use of a Fokker-Planck equation for simulating dechanneling process in thick single crystals. In this paper, we have used a numerical method based on the Fokker-Planck equation to the simulation of spectral distribution of the radiation in planar channeling. Dechanneling effect for electrons (positrons) in (1 0 0), (1 1 0) and (1 1 1) planes of Silicon, Diamond, Germanium and Tungsten crystals with the advantage of the numerical solutions of the Fokker-Planck equation have been studied. The effect of dechanneling on spectral intensity of CR for electrons (positrons) in diamond structures and tungsten single crystals is studied. The dependence of CR intensity on the incidence angle of particles is also considered. Moreover, in completing our previous research for calculating the channeling radiation spectra, both dipole and non-dipole approximations have been applied together with dechanneling processes in major planes of Silicon, Diamond, Germanium and Tungsten crystals. The simulation calculations are used to get information on dechanneling lengths of considered crystals. Finally, we described our Mathematica package that users can use to simulate spectral-angular distributions and energy spectra of planar CR of relativistic electrons (positrons) channeled along the major planes of a Si, C, Ge and W single crystal taking into account the effect of dechanneling process in the crystals. The package is designed to solve the Fokker-Planck equation numerically in order to take into account dechanneling in spectral-angular distributions and energy spectra of channeled relativistic electrons (positrons). The advantage of our presented package is a realistic simulation of spectral-angular distribution and energy spectrum of planar CR.