Crude oil is becoming more and more difficult to refine because of increasingly large heavy oil molecules. Increased external surface of catalysts contributes to promoting precracking of heavy or extra-heavy oil molecules. Flowerlike hierarchical Y zeolite with considerably increased external surface was synthesized without using any organic templates by a hydrothermal procedure. Because the isolated nanoparticles are unstable, the primary nanocrystals gather via self-assembly into loose aggregates. The inner crystals, which act as the "pistils", are difficult to grow because of the confined spaces, while the outer crystals in the aggregates can further grow and form the oriented sheet "petals". The increased exterior surface offers the catalysts dramatically elevated conversion when tested in the catalytic cracking of triisopropylbenzene. The result also suggests that the activity of the catalysts in large reactant involved reactions may generally depend on its external surface properties.