Recent interest in the latent heat energy storage system may be attributed both to reducing energy consumption and to correcting the gap between on-peak and off-peak electrical power demand. Paraffin is suitable material from the viewpoint of high latent heat and various melting points. Because of the low thermal conductivity of paraffin, we have already suggested that the effective thermal conductivity of paraffin was enhanced by mixing metal fiber materials to paraffin. In our previous paper, the heat release characteristic of paraffin with metal fiber materials was improved by enhanced effective thermal conductivity under the constant lower cooling temperature condition. But the mixed metal fiber materials prevented the natural convection flow of liquid paraffin on the heat storage process under the constant lower heating temperature condition. In this paper, the influence of a metal fiber diameter on the heat storage and release process of paraffin with metal fiber materials has been investigated. As a result, it was found that the strength of natural convection of liquid paraffin depends on the metal fiber diameter and the volumetric fiber fraction.