In this study, the multiwalled carbon nanotube-poly (aniline) (MWCNT-PANI) composite is successfully deposited on the glassy carbon electrode (GCE) by electropolymerization as an efficient and conventional route. The structural and chemical properties of the MWCNT-PANI was investigated by various spectroscopic and analytical techniques, scanning electron microscopy (SEM), X-Ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). After that, the electrochemical oxidation behavior of serotonin (5-HT) in phosphate buffer solution (pH 7.4) was examined using cyclic voltammetry (CV). Besides, the MWCNT-PANI/GCE used for the detection of 5-HT with linear concentration range (1.0x10(-7)-1.2x10(-5) mol L-1), low limit of detection (3.3x10(-8) mol L-1) under differential pulse voltammetric (DPV) conditions. The proposed electrochemical sensor based on MWCNT-PANI is both time and cost effective, has good reproducibility, high selectivity as well as stability for 5-HT determination. The developed composite electrode was used to detect 5-HT in synthetic urine sample and satisfactory results were obtained with high recoveries.