A magnesia-based beta alumina sintered body having the following features: (1) the average crystalline particle diameter of beta alumina crystals as calculated assuming that the beta alumina crystals are of a circular section is in a range of 1-4 .mu.m; (2) the amount of the beta alumina crystals having particle diameters not more than 5 .mu.m is 85% to 98% when measured in a plane; (3) the maximum crystalline particle diameter is not more than 300 .mu.m, and the number of coarse particles having diameters falling in a range of 100 .mu.m to 300 .mu.m is not more than 1 as counted in an area of 10 mm.times.10 mm; and (4) the content of crystals of sodium aluminate is 0.5 wt % to 6.0 wt %. A process for producing such a magnesia-based beta alumina sintered body is also disclosed.