Abstract: 12 microsatellite markers from 4 chromosomes were chosen to analyze the effect on 5 milk production traits (including fat, lactoprotein, lactose, dry matter content and somatic cell number) of 150 cows from a 6-male-line Simmental breeding core group. Results showed that the 12 microsatellite markers were highly polymorphic and their PIC (polymorphism information contents) were over 0.60, and the marker with the highest PIC (0.85) was ILST093. The heterozygosities were among 0.64 and 0.86. Effects of 12 microsatellite loci on milk production traits were analyzed. Results showed that ILST093 had a significant effect on somatic cell number (P<0.05). Effects of BMS711 on fat ratio and of BMS1905 on lactose both reached a significant level (P<0.05), BM6438 on milk production traits had no significant effect.