Modified polytetrafluoroethylene fine powder and process for preparing the same

A process for preparing a modified polytetrafluoroethylene fine powder comprising the step of copolymerizing tetrafluoroethylene and a fluoroalkyl vinyl ether in an aqueous medium in the presence of a water soluble fluorine-containing dispersant, wherein at least one fluoroalkyl vinyl ether selected from the group consisting of a compound of the formula:

• X(CF2)nOCF═CF2 (I)
• wherein X is a hydrogen atom, a fluorine atom or a chlorine atom, and n is an integer of 1 to 6, and a compound of the formula:
• C3F7(OCF2CF2CF2)m[OCF(CF3)CF2]kOCF═CF2 (II)
• wherein m and k are independently an integer of 0 to 4, provided that the sum of m and k is not 0 is added to a reaction system at latest at a start of a reaction and a copolymerization reaction of said fluoroalkyl vinyl ether with tetrafluoroethylene is initiated, and after copolymerization is initiated, chain transfer agent is added to the reaction system, but no chain transfer agent is added to the reaction system until at least 80% of tetrafluoroethylene to be polymerized is consumed.