a process of producing a perfluorocarboxylic acid fluoride having the formula : ## str1 ## wherein n is an integer of at least 2 , comprising effecting oligomerization of hexafluoropropylene oxide in the presence of an alkali metal fluoride a sulfone or a mixture of a sulfone with an ether having the formula : ch . sub . 3 o . sub . m ch . sub . 3 where m is an integer of 2 to 6 . this process makes it possible to improve production yield of hexafluoropropylene oxide oligomers remarkably , and to raise a reaction temperature for the oligomerization , as compared with conventional methods , thereby shortening reaction time therefor .