A new sensor based on carboxyl-functionalized mesoporous molecular sieve/colloidal gold modified nanocarbon ionic liquid paste electrode (MCM-41-COOH/Au@nano-CILPE) was developed for electrochemical determination of serotonin (5-HT). The carboxyl-functionalized mesoporous molecular sieve was characterized by FTIR spectroscopy. The electrochemical behavior of 5-HT was studied using square pulse voltammetry (SWV). Compared with the traditional carbon paste electrode, the current response of 5-HT was significantly increased on the modified electrode. Under the optimal experimental conditions, the developed sensor showed a linear response in the concentration range of 0.2-20 mu M with a detection limit of 0.1 mu M for 5-HT. The method was successfully applied for the determination of 5-HT in human blood serum samples.