Electrochemical detection of nucleic acid base mismatches related to Apa I single nucleotide polymorphism (SNP) in the vitamin D receptor gene was performed successfully using 7-dimethyl-amino-1,2-benzophenoxazinium salt (Meldola's blue, MDB) with 10.9 pmo1/100 mu L of detection limit. MDB reduction signals obtained from probe, mismatch(probe-SNP containing target) and hybrid(probe-target) modified pencil graphite electrode(PGE) increased respectively. The sensor was able to clearly distinguish perfect match from mismatch DNA in a 30 min. detection time. Several factors affecting on the hybridization and indicator response are studied to maximize sensitivity and selectivity. The advantages of the biosensor are discussed in comparison with previous electrochemical assays for DNA hybridization.