The on-chip genotyping system ("the electrochemical DNA chip") has been developed as a more cost-effective genotyping system and was applied to MDR1 genotyping in the present study, which is required for wide use in clinical application and for personalized medication based on genotype. The electrochemical DNA chip was optimized and applied to simultaneous genotyping of four MDR1 polymorphisms (T-129C, C1236T, G2677(A,T) and C3435T) using synthetic model oligonucleotide DNA and human genomic DNA. The electrochemical DNA chip successfully gave the T-129C, C1236T, G2677(A,T) and C3435T genotypes, which were completely consistent with those determined by direct sequencing. In conclusion, the electrochemical DNA chip is useful for simultaneous determination of some genotypes and haplotypes, and efficient genotyping using this system can support future genotype-phenotype studies at a large scale.