Abstract: The purpose of this study was to investigate effect of FMR1 gene knockout on productive performance of C57BL/6 mice. The gene knockout heterozygous mice were fed in SPF environment for propagation according to principles of genetics. The genomic DNA from tail tissue of each genotype mice was extracted to identify their genotypes using PCR technology. The results showed that PCR technology could detect genotypes in mice, and was convenient, intuitive and reliable. There were 3 genotypes (wild type, heterozygous and homozygote) in offspring of FMR1 gene knockout heterozygous mice. On the basis of gene identification, 40 offsprings of C57BL/6 FMR1 KO and C57BL/6 (20 male and 20 female mice, aged 10 weeks) were chosen and monogamously mated. After natural delivery, the research tested the reproductive performance and done statistical analysis. Reproductive performance indices of C57BL/6 FMR1 KO mice, such as litter size, weaning rate, body weight and body length of neonatal mice, body length of weaning mice and rate of male and female, had relative low values compared with C57BL/6 mice, but were not significant(P>0.05). However, there was extremely significant difference between C57BL/6 FMR1 KO and C57BL/6 mice in body weight of weaning mice.