Abstract: This study established a quantitative RT-PCR method with primers T3-4F/R based on the conserved nucleotide sequence of Citrus tristeza virus（CTV）gene ORF1a. The results showed that the method was at least hundred times higher than that with conventional PCR. A good linear correlation（R2 = 0.992）obtained from two standard curve of cRNA. The amplification efficiency was 97.1%. Three-time repeats revealed that the coefficients of variation between the intra- and inter-assay were both within 2.94%，indicating a reliating reproducibility detection method to CTV. There was noticeable differences among the content of T3 genotype in field samples，the highest content can reach to 1 250 times the lowest. The method was used for accurate determination of T3 genotype in the plant，and could be used to studythe variation of T3 genotype.

Key words: Ctrus tristeza virus（CTV）, T3 genotype, quantitative RT-PCR