In the preparation of a modified polytetrafluoroethylene fine powder by polymerizing tetrafluoroethylene in an aqueous medium in the presence of a water soluble fluorine-containing dispersant, a fluoroalkyl vinyl ether is added to a reaction system at latest at a start of a reaction and a copolymerization reaction of the fluoroalkyl vinyl ether with tetrafluoroethylene is initiated, and a chain transfer agent is added to the reaction system after at least 80% of tetrafluoroethylene to be polymerized is consumed. The obtained modified polytetrafluoroethylene fine powder has good molding properties, in particular, extrusion properties, and high heat resistance.