In this thesis, an electrochemical sensor based on nickel oxide nanoparticles supported on poly melamine formaldehyde-reduced graphene oxide is fabricated for determination of methotrexate concentration. At first, a bare glassy carbon electrode surface was modified by NiO@PMF-rGO nanocomposite. Field emission-scanning electron microscopy, transition electron microscopy, fourier-transform infrared spectroscopy, energy dispersive x-ray spectroscopy and x-ray diffraction were used to characterize nanocomposite. Following that, parameters affecting the sensor response such as pH, time, amount of the electrocatalyst and instrumental parameters were optimized using cyclic voltammetry and differential pulse voltammetry. Under the optimized conditions, calibration curve was drawn and two linear ranges were obtained from 0.02 ?M to 0.8 ?M and from 0.8?M to 6.0 ?M with a detection limit of 0.006 ?M. Repeatability with relative standard deviation of 2.5% and reproducibility with relative standard deviation of 4.6% were obtained. Finally, the sensor was tested in the presence of some potential interferences and in human blood plasma with acceptable response. Recovery percentages of 100.3 and 98.2 were observed.