Superconducting MgB 2 bulk samples were simply sintered by heating stoichiometric mixture of Mg and B in vacuum. In situ resistance measurements were performed carefully in order to find the best annealing process. The in situ resistance measurements exhibit a decrease in resistance when the heating temperature is increased. The measurements suggested that the Mg powder melting temperature was below 650 o C and the temperature of 800 o C was higher than required. A MgB 2 bulk sample sintered at 750 o C has a superconducting transition temperature of T c (R=0)=40.3K, higher than that sintered at 680 or 810 o C.