The time variation of the dielectric constant and the satellite reflections has been measured following the incommensurate-commensurate transition in K2ZnCl4. The dielectric constant approaches the steady value with a exponential function of the square root of time. X-ray diffraction study has revealed that the dielectric relaxation is accompanied by the change of satellite and diffuse scatterings. The slow relaxation is considered to reflect a nucleation kinetics of discommensurations under the influence of crystal imperfections.