The modified electrode was constructed by intercalating carbon nanotubes (CNT) on graphite surface, which exhibits an attractive ability to determine dopamine (DA) and serotonin (5-HT) simultaneously in the presence of ascorbic acid (AA), resulting in a high DPV current response to 5-HT at 0.32 V and DA at 0.13 V and a favorably low response to AA at -0.07 V versus SCE in a 50 mM phosphate buffer (pH 7.4). The properties and behaviors of the modified electrode were characterized using CV and DPV. The results show that the high sensitivity and selectivity is attributed mainly to the unique carbon surface of the nanotubes and the porous interfacial layers that come from the subtle tubule structure of CNT. The linear calibration range for DA in the presence of 0.5 mM AA and 5 muM 5-HT was 0.5-10 muM, with a correlation coefficient of 0.9996, and a detection limit of 0.1 muM. A linear relationship was found for 5-HT in the range of 1-15 muM containing 5 muM DA and 0.5 mM AA, with a correlation coefficient of 0.9998 and a detection limit of 0.2 muM. The current sensitivities were 1.89 and 8.05 muA muM(-1) for 5-HT and DA, respectively. Rabbit's brain homogenate presented no interference to 5-HT and DA simultaneous detection.