Nucleation process is a rate-limiting step of DNA strand exchange reactions between double helical DNA and its complementary DNA. Here we designed the double stranded probes that have single stranded portion. The single stranded portion is responsible for the nucleation process and mismatch recognition. A single base mismatch in 25mer DNAs could be detected in a few minutes at ambient temperatures. Using this method, various types of SNPs are rapidly and reliably detected without any aids of enzymes or a special instrument.