Abstract: In order to obtain a simple,fast,accurate and low-cost diagnosis method of fragile X syndrome,cytogenetic tests and molecular genetic tests were carried out with direct amplification of (CGG)_{n<\sub> repeat sequence in 5^'terminal of FMR1 gene by PCR and the cDNA sequence of FMR1 by RT-PCR from six mental retardation pedigrees.The proband of pedigree A with highexpression of fragile X chromosome(35/273) was detected to be a full mutation patient of fragile X syndrome by the molecular genetic test.There is no expression of fragile X chromosome in the proband and his mother of pedigree B,which was futher confirmed as a non-fragile X pedigree by the molecular genetic test.A male foetus of the pedigree C has fragile X chromosome(5/93),but the proband and his mother has no fragile X chromosome.By further detection using molecular genetic test,the male foetus is a full mutation patient of fragile X syndrome,his mother is a permutated carrier,and his brother is a mosaic patient.The proband of pedigree D has high expression of fragile X chromosome(17%),his sister also has expression of fragile X chromosome(5%).By further detection with molecular genetic test,the proband is a full mutation patient of fragile X syndrome,and his sister is a mosaic patient.The probands of pedigrees E and F of the mother were found with suspicions fragile X chromosome,being confirmed as the non-fragile X pedigrees by the molecular genetic test.The conclusion is that the analysis test with direct amplification of 5^'(CGG)n<\sub> repeat sequence and cDNA sequence in FMR1 gene is simple,fast,low-cost and can be applied in screening,diagnosis and prenatal diagnosis of fragile X syndrome.}