Y-zeolite/MCM-48 biporous molecular sieve has been synthesized in the mode of overgrowth of MCM-48 on the surface of pre-added zeolite Y. The as-synthesized sample was characterized by XRD, SEM, TEM, IR and nitrogen sorption methods and the corresponding hydrothermal stability was studied. It is found that the composite material exhibits both properties of mesoporous MCM-48 and microporous Y-zeolite sieves. The DFT pore diameter mainly located at 1.2 and 3.2 nm. In the progress of synthesis, the preadded NaY acts as a seed material and MCM-48 overgrow on the surface of Y-zeolite. Compared with MCM-48 molecular sieve, this new biporous molecular sieve shows a higher hydrothermal stability, which results in its thicker pore-wall, and stronger acidity. (C) 2008 Elsevier B.V. All rights reserved.