Abstract The heat transfer characteristics of phase change material slurries, e.g. tetra-n-butyl ammonium bromide (TBAB) clathrate hydrate slurry (CHS) and microencapsulated phase change material (MPCM) slurry, flowing through the heated circular tubes under constant heat flux are investigated in the present paper. The continuity equation, momentum and energy equations for the phase change material slurry in tubes are solved to obtain the variation of the temperature of the phase change material slurry with time and along the flow direction. The temperature variation of TBAB CHS can be divided into two regions, while there are three regions for MPCM slurry due to the existence of supercooling state. The comparison between the calculated and measured results reveals that MPCM slurry is possibly not fully melted during the flow melting, leading to a shorter melting region and higher outlet temperature than those of calculated values based on the heat balance equations.