We report here the preparation of biodiesel by transesterification of rapeseed oil with methanol using calcined K 2 CO 3 /c-Al 2 O 3 as a solid base catalyst. The prepared catalysts were characterized using SEM, IR and BET, and their catalytic activities were evaluated. The reaction conditions were optimized, and in particular, the conversion can be as high as 98.62% under the optimal reaction conditions. In addition, the effect of the presence of water in the reaction system on the catalytic activity was also studied.