This paper aims to clarify the optimum grinding condition for fine grain diamond wheels in precision grinding of fine ceramics. A series of plunge surface grinding tests on alumina ceramics have been conducted using fine grain diamond wheels with grain size in the range of 2-6to 5-12μm. On the basis of these results, the optimum grinding condition has been evaluated from a veiw point of grinding ratio. The main results obtained in this paper are as follows : (1) When the grinding speed does not exceed about 1 800m/min, the grinding ratio is solely a function of coefficient of grain depth of cut φg= (v/V) √t/D (v; work speed, V; peripheral wheel speed, t; wheel depth of cut, D; wheel diameter) approximately. (2) When φg exceeds a critical value φgc, a transition from attrition to fracture type in the wear mode of diamond wheel occurs. Therefore, the grinding ratio takes a maximum value at the critical condition φg=φgc. The critical Value φgc decreases in proportion to the mean grain diameter of diamond abrasives.