Abstract: To investigate the molecular mechanism of Melanocortin-4 receptor（MC4R），PCR-SSCP was used to detect the polymorphism and analyze the bioinformatics of gene MC4R exon in 3 sheep populations（Gansu meat sheep new breeding population，Small Tail Han sheep，and Hu sheep）. The results showed that 3 genotypes of AA，AB and BB existed in 3 populations，and the dominant genotype was BB，the preponderant allele was B. The sequencing analysis indicated that comparing wild BB and mutated AB，G→A mutation in locus 511 and T→C mutation in locus 495 happened in AB；G→A mutation in locus 511 with AA homozygosity and T→C mutation in locus 495 with CC homozygosity happened in AA. The PIC of Small Tail Han sheep was moderately polymorphic（0.25 < PIC < 0.50），Gansu meat sheep new breeding population and Hu sheep belonged to low polymorphism（PIC < 0.25）. Except the Hu sheep，the other 2 sheep breeds were in Hardy-Weinberg equilibrium by χ2 test. Bioinformatics analysis showed that the amino acid sequence of MC4R had hydrophobic region significantly，7 transmembrane helical regions and signal peptide. The main elements in the second structure of the encoded protein were the α helix and random coil. Homology analysis demonstrated that the similarity of gene MC4R in sheep with goat，cattle，pig，human，and gorilla was 97%，94%，81%，83% and 83% respectively，indicating that MC4R was an evolutionarily conserved protein，and played an important role in growth o f sheep.

Key words: sheep, gene MC4R, polymorphism, bioinformatics