The present block copolymer contains a structural unit (A) derived from a compound represented by the general formula (I): CH.sub.2 .dbd.CR.sub.1 COOR.sub.2 R.sub.f, and also contains a structural unit (B) consisting of a polymer moiety represented by at least one of the structural formulae (III) and (IV) and another polymer moiety derived from a compound represented by the general formula (II): CH.sub.2 .dbd.CR.sub.3 R.sub.4. For preparing the above block copolymer, a block copolymer consisting of the structural unit (A) derived from a compound represented by the general formula (I): CH.sub.2 .dbd.CR.sub.1 COOR.sub.2 R.sub.f and the structural unit (B1) derived from a compound represented by the general formula (II): CH.sub.2 .dbd.CR.sub.3 R.sub.4 and a compound represented by the general formula (V): CH.dbd.CR.sub.6 COO(R.sub.9).sub.t H are reacted with a compound represented by the general formula (VI): R.sub.10 COCl to allow a dehydrochlorination reaction to take place. Thus, an unsaturated group and fluorine containing block copolymer having excellent water and oil repellent properties and excellent sensitivity to activation energy rays can be prepared. The block copolymer is utilized as a surface modifier for activation energy-curing resins.