A theoretical study has been made to investigate the nucleation kinetics of cadmium sulphide (CdS) thin film grown by CVD technique. In CVD technique, the temperature gradient between the source zone and the growth zone provides the necessary driving force for the nucleation of CdS thin film over the substrate. The incident flux at the substrate depends mainly upon the temperature gradient between the two zones. In the present work, the equilibrium flux is estimated from the activation energy and heat of reaction due to the reaction and diffusion in the medium and the condensation at the substrate. The nucleation rate and the growth rate of the film have been calculated for different temperature gradients and compared with the available experimental results.