SNP in exon 8 of the Single-minded 1 (SIM1) gene was detected using PCR-SSCP in 169 pigs of 6 pig breeds including Yorkshire, Duroc, Pietrain, Landrace, Jiaxinghei and Jinhua. The results showed three genotypes (CC, CT, TT) were found and their frequencies between domestic and foreign breeds were different. Only one genotype TT was found in Jiaxinghei and Jinhua pigs, but three genotypes were detected in each foreign breed. The effects of this SNP on backfat thickness in Landrace, Yorkshire and Duroc were estimated using least square method. The homozygote pigs had thicker backfat than the heterozygote ones. The significant effect of various genotypes was found in foreign breeds (P < 0.05). SIM1 had different effects on backfat thickness of different points.