A process for alkylation of an isoparaffin with an olefin comprising contacting an isoparaffin containing feed with an olefin-containing feed in the presence of a catalyst, said catalyst is a supported heteropoly acid catalyst, which is represented by the following chemical formula: H.sub.k YW.sub.m Mo.sub.12-n O.sub.40.multidot.nH.sub.2 O, wherein k is 3 or 4, Y is an atom of P, Si, Ge or As, m is a positive integer between 0-12, n is any number in the range of >0-10; the reaction temperature is at least equal to the critical temperature of isoparaffin and up to 300.degree. C.; the reaction pressure is at least equal to the critical pressure of isoparaffin. The advantages of this process are embodied in high olefin conversion, high alkylate yield, and good retention of catalyst stability at the same time.