It may be important to elucidate the mechanism of grain growth during the early stage of sintering, especially for micro-scale sintered products, of which size is close to that of powder particles. This study aims to clarify the grain growth behavior in powder compacts, that is, porous bodies. Sintering experiments of the powder compacts with polycrystalline nickel particles were conducted in temperatures lower than usual, for long time, and the change in crystal grain size were revealed by electron backscatter diffraction analysis in a scanning electron microscope. It was found that the crystal grain size increased, but did not go over the size of powder particles in the case of sintering at the lower temperatures. A phase-filed simulation of sintering process for two polycrystalline particles was also performed to examine the effects of neck growth on grain boundary migration. It was confirmed that the two polycrystalline particles of equal-size became two single-crystal grains when the sintering shrinkage rate was set to be low, which may be correspond to the experimental results at the lower temperatures.