A polycarbonate-polydimethylsiloxane copolymer, the copolymer comprising a polycarbonate block of the formula ##STR1## wherein R.sup.1 and R.sup.2 are hydrogen or a C.sub.1 -C.sub.4 alkyl, R.sup.3 and R.sup.4 are hydrogen, halogen, a C.sub.1 -C.sub.20 alkyl or an aryl, x is 1 to 5, y is to 4 and n is 1 to 100, and a polydimethylsiloxane block of the formula ##STR2## wherein R.sup.5 and R.sup.6 are an organic residue having an aromatic nucleus and m is greater than or equal to 100, the copolymer having a content of the polydimethylsiloxane block of 0.5 to 10% by weight, an n-hexane soluble matter content in an amount of 1.0% by weight or less and a viscosity average molecular weight of 10,000-50,000. The copolymer has a high impact resistance and when blended with a glass fiber or a carbon fiber forms a molded product having an extremely high Izod impact strength.