The preliminary cracking (pre-cracking) of heavy oils has been studied on a typical fluid catalytic cracking catalyst. Contributions of several factors were evaluated, including those of zeolite, thermal reactions, and active matrix. Results indicate that zeolite (REUSY) plays a leading role in pre-cracking and its contribution always accounts for more than 50%. In highlighting the remarkable effect of zeolite, a new hypothetical reaction route, namely the relay of active sites, is proposed for the consideration of zeolite internal surface. The contributions of thermal reactions and active matrix change with the contents of aromatic components in different feeds. The active matrix is effective in pre-cracking of large hydrocarbon compounds with aromatic rings; its contribution increases with the aromaticity of the feed. For naphthene- or medium-based feeds, the active matrix can boost pre-cracking significantly, and too many thermal reactions are disadvantageous in this case. For paraffin-based VGO, excessive matrix activity will result in the over-cracking of desired products. (C) 2013 Elsevier Ltd. All rights reserved.