We have successfully diagnosed 24 individuals of 5 haemophilia A pedigrees with RFLP linkage analysis and gene amplification in vitro (PCR) technique. Detecting with the I14-E18, we found the carriers of family A and B were not heterozygotes of Bcl I RFLP. They were homozygotes of 3.0 kb/3.0 kb and 2.3 kb/2.3 kb respectively, while the carriers of family C and D were heterozygotes of 3.0 kb/2.3 kb and 3.3 kb/2.3 kb respectively. So it could be made gene diagnosis with Bcl I RFLP in family C and D. In family D, pathologic gene was linked to 3.3 kb band. We have exactly made gene diagnosis to family A and B with RFLP linkage analysis after PCR. Our results showed that the carriers of family A and B all were heterozygotes of 142 bp/99 bp, In family A and B, pathologic FVIII gene was linked to 142 bp fragment. With Bgl II/DX13, we made a RFLP linkage analysis of haemophilia A fetus at the early pregnant period, whose mother was heterozygote of 5.8 kb/2.8 kb, and the fetus was hemizygote of 2.8 kb/-. So it was very clear that the fetus was patient of haemophilia A. We suggested that the pregnant woman to induce abortion to prevent a sick baby from being born.