A distinct bimodal microstructure has been obtained in a Si3N4-BAS (barium aluminum silicate) ceramic-matrix composite by pressureless sintering, It is shown that the addition of coarse beta-Si3N4 seeds causes abnormal grain growth in this composite, and hence encourages the formation of a bimodal microstructure. This abnormal grain growth is due to the nature of the heterogeneous nucleation mechanism in Si3N4 alpha-to-beta phase transformation, and is promoted by the transformation. After complete phase transformation, further abnormal grain growth is comparably slow and governed by the Ostwald ripening mechanism. Therefore, a stable bimodal microstructure can be easily achieved by pressureless sintering.