A very sensitive electrochemical sensor has been developed by modification of glassy carbon electrode (GCE) with nanoparticles of bismuth (III) oxide (Bi2O3) and multi-walled carbon nanotubes (MWCNTs). The sensor was applied for the determination of cilostazol, cyclic nucleotide phosphodiesterase inhibitors in pharmaceutical formulation and human plasma. The voltammetric responses were compared with those obtained at bare GCE under optimum conditions. The cyclic and square-wave voltammograms of cilostazol showed 3.3 and 4.9 times enhancement in the oxidation peak current at MWCNTs-Bi2O3/GCE as compared to a bare GCE. Bi2O3-MWCNTs/GCE showed a linear response for cilostazol in standard solution over the concentration range of 0.8-13 mu g mL(-1) with the detection limit 0.76 mu g mL(-1), whereas human plasma over the concentration range 0.8-12.5 mu g mL(-1) with the detection limit 0.66 mu g mL(-1).