A bi-functional ionic liquid 1-(benzotriazole-1-methylene)-3-methylimidazole bis(2-ethylhexyl) phosphate ([BTAMIM][DEHP]) was prepared. It exhibited high activity and selectivity for the esterification of pentaerythritol with caproic acid to pentaerythritol tetra-hexanoate (PETH). Meanwhile, the friction reduction and anti-wear performance of pentaerythritol tetra-hexanoate were improved significantly by [BTAMIM][DEHP].