Objective To develop a methylated plasmid control of promoter 1A of adenomatous polyposis coli (APC),and control the sensitivity of methylation specific PCR (MSP) of clinical samples.Methods Methylated and unmethylated PCR products were cloned into plasmid pMD18-T.Liner plasmid for lowest sensitive control (cut-off control) of MSP was quantified with the method of UV absorption and quantitative MSP was carried out using SYBR Green I.Thirty-six samples were detected with the improved MSP.Results The methylated and unmethylated plasmid controls of promoter 1A of APC were made.The sensitivity of FQ-MSP was 104 copy /ml and the cut-off control of MSP was 105 copy /ml.With the weak positive cut-off control,two negative samples detected formerly were confirmed as positive;uncertain 11 samples were confirmed as 7 positives and 4 negatives.The methylation rate of 36 samples was 50%.This result showed very significant difference compared to the methylation rate of 25% without the cut-off control (P=0.001).Conclusion The methylated plasmid cut-off control of promoter 1A of APC was developed,which can be used to make the MSP results more reliable.