Mu opioid receptor (MOR) is a member of G protein-coupled receptor family, distributed in the pain transduction region in the brain and related to emotion and behaviour. This study was designed to investigate the Single Nucleotide Polymorphism (SNP) of Mu opioid receptor gene in various breeds,including duroc, landrace and Yorkshire. 5' UTR (untranslate region), coding region and 3' UTR of Mu opioid receptor gene were amplified by eight pairs of primers, and the Single Nucleotide Polymorphism (SNP) were detected by SSCP. Five polymorphisms were found (Genebank Accession number: AF521309). The results of chi(2) test showed that the frequencies of genotypes in different breeds were significantly different (P<0.01). The frequencies of mutation genotypes in Yorkshire were significantly higher than Duroc and Landrace. According to the above results, we can speculate the difference of the frequencies of genotypes may be the results of long term choice pressure.