A fixing structure of a faucet is mounted on a sink countertop with an opening and contains a base including a supporting portion and an inserting portion. A supporting portion has an annular fence and an annular shoulder; the annular fence has a retaining recess; the annular shoulder has two longitudinally through holes and an opening. The inserting portion has a cylindrical fence and two stopping rods, and the cylindrical fence has a longitudinal channel; the annular shoulder has an annular abutting area. A locking assembly includes two positioning screws and two clamping blocks. A faucet includes a fitting tube, a sheath having a groove, a peripheral fence, and a threaded orifice. A controlling valve assembly is mounted on the fitting tube. A fastening bolt is screwed with the threaded orifice so as to retain with the retaining recess, thus fixing the faucet on the base.