The boron-doped conductive diamond electrode(BDD) was used as a working electrode for detection of serotonin, (5-hydroxytryptamine) (SE) in the presence of ascorbic acid(AA) which is mainly contained from vitamin C in human blood. In this study, the cationic polymer film of N,N-dimethylaniline (DMA) was electrochemically deposited on the BDD electrode surface to modify the characteristics of electrode. The PDMA film-coated BDD electrode can separate the oxidation potentials of AA and (SE) which coexist in a homogeneous solution. The selective detection of SE is achieved at the PDMA film-coated BDD electrode in the presence of AA.