Many concerns have been raised concerning the mechanism whereby the thickness of electrodes in molten carbonate fuel cells is reduced over time. We reported in a previous paper that when CO2 partial pressure in the atmospheric gas is low and the temperature of molten carbonates is high, porous nickel oxide deforms easily under compressive stress. In this paper, the mechanism of cathode deformation, which has not yet been clarified, was investigated. Results indicate that cathode deformation occurs when particles in a highly stressed porous nickel oxide dissolve and then reprecipitate to adjacent particles which are less stressed.