A numerical investigation on natural convection heat transfer in a 2-dimensional enclosure heated from below has been conducted with microencapsulated phase change material (PCM) slurries. The microencapsulated PCM slurry exhibits pseudoplastic non-Newtonian fluid behavior and has a peak value in the specific heat capacity. The viscosity of the slurry is assumed to follow the Ostwald-de Waele power law fluid model with the power-law index n and the consistency coefficient K. Both dynamic and heat transfer characteristics are discussed on Rayleigh-Bénard natural convection. Characteristics on natural convection heat transfer have been numerically investigated for different value of the width-height aspect ratio of the enclosure. According to the comparison of the two kinds of fluid with PCM and without PCM, the start of natural convection is earlier for the fluid with PCM.