In order to reduce phase change time in latent heat technology, improvement of effective thermal conductivity of heat storage unit would be one of the techniques. Effect of effective thermal conductivity on melting time is studied analytically of circular composite heat storage capsules made by immersing phase change materials (PCM) into porous metals. Numerical and approximate analysis were made with the consideration of uniform and non-uniform heat transfer coefficients around the cylindrical surface. Four PCMs (H₂O, Octadecane, Li₂CO₃, NaCl) and three metals (copper, aluminum and carbon steel) were selected as specific materials. Porosities of the metals were restricted larger than 0.9 in order to lessen decrease in latent heat. Results show that reduction in melting time was obtained for the above PCMs, especially for low conductivity PCMs, Melting time obtained by approximate analysis agrees well with numerical analysis. High Nusselt number and high thermal conductivity of heat transfer fluid would be more effective to reduce phase change time.