A convenient, sensitive, and label-free method to determine the DNA methylation status of CpG sites of plasmid and human colon cancer cell has been developed. The system relies on highly selective single base extension reaction and significant optical amplification of cationic conjugated polyelectrolytes (CCP-1). The higher fluorescence resonance energy transfer efficiency between CCP-1 and fluorescein-labeled dGTP (dGTP-FI) is correlated to the incorporation of dGTP-FI into the probe DNA by single base extension reaction when the target/probe pair is complementary at the methylation site. As low as 1% methylation status can be determined by this new assay method. Because of the optical amplification property of CCP-1, the method exhibited high sensitivity with a concentration of analyte DNA at the picomolar level. The CCP-1 can form a complex with negatively charged DNA through electrostatic interactions, avoiding labeling the DNA target and probe by covalent linking. The isolation steps employed in other typical assays were avoided to simplify operations and increase repeatability. These features make the system promising for future use for early cancer diagnosis.