We show that laser cooling by coherent population trapping in case of zero radiation force — when atomic evolution is due to only momentum diffusion — can be effectively described by use of the Fokker-Planck equation for the atomic distribution function. Our quasiclassical approach allows numerically and analytically to investigate the dynamics and the efficiency of the cooling process. It is shown that both the evolution of the momentum distribution and the limit of the cooling are determined by the degree of incompleteness of coherent population trapping.