The kinetics of a thin layer Pd \ H(D) electrode at the coexistence of alpha and beta phases in cyclic voltammetry are studied and the effects of various parameters are discussed. It is found that the voltammogram of the alpha<->beta phase transition is trigonal in shape which differs significantly from those for diffusion and adsorption. The kinetic characteristics of the alpha+beta mixed region are controlled by the deviation of parameters from the thermodynamic values. These results indicate that cyclic voltammetry can be used for studying the kinetics of phase transition occurring in a layer electrode. The present treatment is verified by comparison with the previous experimental results.