Objective:To study the feasibility of detecting free-fetal DNA in maternal plasma by multiplex short tadem repeat(STR) loci amplification. Methods:Plasma total DNA extracted from 10 medium pregnancy women by improved DNA isolation method were detected to analyze genotype by multiplex STR amplification. Then the extraction efficiency and rate of assay was analyzed using DNA-PAGE(polyacrylamide gel electrophoresis) in comparison with the maternal and fetal genome DNA. Results:Of all the 60 loci detected in 10 medium pregnancy women plasma,44 loci have results. Except the 14 loci which maternal and fetal DNA genetypes were completely same,free-fetal DNA in maternal plasma were detected in 18 of 30 STR loci,the rate of detection was 30%. D13S317 and D12S391 have the maximal detection rate. Conclusion:Multiplex STR loci amplification can detect free-fetal specificity genotype in maternal plasma,which may have expectation for noninvasive prenatal diagnosis of certain hereditary diseases.