Synthesis of mesoporous materials with a bimodal pore size has always been of great importance for size-selective separation process. Especially, ordered mesoporous cubic MCM-48 with its highly interwoven and branched pore structure, is a potential separation material in applications similar to which zeolites are now being used. This material could have specific technological advantages because of the 3-D channel network of MCM-48. We report that by post-synthesis alumination of siliceous MCM-48 containing organic templates with sodium aluminate solution, a bimodal pore size of MCM-48 is obtained. In particular, it was found from N2 adsorption−desorption measurements that MCM-48 obtained had 26 Å and 38 Å dual narrow pore size distributions. From 27Al solid-state MAS NMR measurements, it is confirmed that the aluminium is in the framework structure of mesoporous MCM-48.