A new synthetic procedure for the preparation of perfluoroalkanesulfonyl fluorides utilizing liquid-phase direct fluorination with elemental fluorine has been developed. Direct fluorination of a partially fluorinated ester, which has alkanesulfonyl fluoride in the end, was synthesized from non-fluorinated counterparts and perfluorinated acid fluoride according to the PERFECT process, gave the desired perfluorinated product in moderate yield as well as by-products arising from CS bond cleavage. The results of the direct fluorination of some model substrates suggest that the CS bond cleavage occurred due to radical formation at the α-position rather than the β-position.