Alkylation reaction on a zeolite catalyst was conducted in supercritical phase isobutane or isopentane, which was a reactant as well. Compared to the reactions conducted in a liquid phase or gas phase, the supercritical phase alkylation reaction exhibited higher activity, along with an obviously longer lifetime, on a Y-type zeolite catalyst. Lewis acidic sites were found to be the active sites for the target reaction. High-molecular-weight olefins formed through oligomerization reaction readily deactivated the Lewis acidic sites, as in the liquid-phase or gas-phase reaction. Supercritical fluid was able to extract in situ and transport these high-molecular-weight olefins to extend catalyst life successively. La3+-modifed Na-Y zeolite catalyst was found to be active and stable in the supercritical phase alkylation reaction.