MnZn ferrites have been widely used as magnetic core materials. It is well known that Ca addition is effective to obtain homogeneous microstructure of fine grains and highly resistive grain boundaries. However, the behaviors of calcium as one of the main additives at different temperature ranges during sintering process are not completely understood yet. In this study, the influence of CaCO3 content and sintering temperature on the microstructure was investigated in 1473-1623 K. It was found that there existed a critical temperature around 1550 K. The grain size decreased with the increase of Ca content when the sintering temperature was lower than the critical temperature, but completely opposite result was observed at higher temperatures range. Possible mechanisms were discussed. When the sintering temperature was lower than the critical temperature, Ca content greatly affected the grain boundary mobility and dominated the grain growth. At higher temperatures, however, formation of liquid phases might be the main cause for the grain growth.