Ethylene oligomerization reaction on H-ZSM-5 zeolite catalyst was conducted in supercritical phase n-pentane, Compared with the reactions conducted in the gas phase, the supercritical phase ethylene oligomerization reaction exhibited higher activity, along with longer lifetime. High-molecular-weight olefins formed through oligomerization and cracking reactions readily deactivated the active sites, as in the gas-phase reaction, Supercritical n-pentane was able to extract high-molecular-weight olefins in situ to extend catalyst life successively.