In the present study, the solidification and melting of water were investigated experimentally for the case of a vertical heat transfer plate with pin fins. In the experiment, temperature distributions, ice and water volume fractions, and heat flux changes were measured and the flow patterns in the water were observed for examination of the phase change process. In the solidification, the phase change rate increased monotonously with increasing number of fins. In the melting, the temperature distribution in water showed a uniformity caused by natural convection based on the density change of water. The contribution of the natural convection to the melting was examined based on the relationship between the modified Nusselt number and the Rayleigh number.