In this study, the effect of calcination temperature and time on the structure of calcinated magnesite was investigated. In addition, the effect of calcinated magnesite on rheology when added into bentonite suspension was also examined. Electrokinetic, rheological and stabilization studies including coagulation and gelation tests were performed to explain the mechanism of viscosity increase in bentonite suspension upon addition of the calcinated products. FTIR, SEM, BET analyses and rheological data were used to determine the transition degree of MgCO₃ to MgO, and evaluate the structure of MgO and bentonite suspensions. The results from this study showed that specific surface area (SSA) and mean pore radius (MPR) values for the calcinated products were directly affected by the calcination temperature and time. According to the ignition loss and transition ratio from MgCO₃ to MgO of the calcinated samples, it was found that the calcination was completed above 800^oC. In order to obtain a bentonite suspension with higher viscosity, interaction of the sufficient amount of MgO particles with bentonite particles were necessary. Based on these results, the formation of gelation and heterocoagulation between MgO and bentonite particles were found to be responsible for these effects.