In this thesis, a new electrochemical sensor containing multiwall carbon nanotubes with K 5 [PW 11 CoO 39 ]@Thionine was designed on the glassy carbon electrode surface for determination of morphine. Designed sensor for morphine electrochemical oxidation in Briton-Robinson buffer pH=6.0 showed good electrocatalytic activity. It should be noted that K 5 [PW 11 CoO 39 ]@Thionine led to an increase in the sensitivity of the corrected electrode. Methods such as, Uv-visible spectroscopy, X-ray spectroscopy (EDX)، X-ray diffraction (XRD), Atomic Force Microscopy (AFM), and Field Emisson Scanning Electron Microscopy (FE_ SEM ) were used to conduct morphological investigations and to determine the characteristics and structure of the sensor. Voltammetric cyclic and differential pulse voltammetry techniques were used to study the electrocatalytic ability of the above mentioned sensor for morphine. The linear ranges obtained for the detection of morphine is 0.02 to 1.00 and 1.00 to 50.00 µmol L -1 . The reported detection limit was also 2 nM. Finally, the electrode was used for determination of morphine in urine and plasma samples.