This paper has dealt with melting heat transfer characteristics of ice water slurry in an inside tube of horizontal double tube heat exchanger in which hot water circulated in an annular gap between the inside and outside tubes. In this study, two kinds of heat exchangers were used; one is made of acrylic resin tube for flow visualization and the other is made of stainless steel tube for melting heat transfer measurement. The result of flow visualization revealed that ice particles flowed along the top of inside tube in the ranges of small ice packing factor and small ice water slurry velocity, while ice particles diffused into the whole of tube and flowed like a plug built up by ice particles for large ice packing factor and high velocity. Moreover, it was found that the flowing ice plug was separated into numbers of small ice clusters by melting phenomenon. Experiments of melting heat transfer were carried out under some parameters of ice packing factor, ice water slurry velocity and hot water temperature. Consequently, the correlation equation of melting heat transfer was derived as a function of those experimental parameters.