This article presents the novel modifier; polyacrylonitrile nanofibers/Ni0.5Zn0.5Fe2O4 nanoparticles nanoparticles as a novel modifier, which has been synthesized using an electrospinning method. This nanocomposite has been used to construct an electrochemical sensor for highly sensitive determination of sunitinib (STB) drug in biological samples. The synthesized nanocomposite was fully characterized using methods such as Fourier transform infrared spectroscopy, scanning electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. For the electrochemical determination of STB, the differential pulse voltammetry signal of the drug electrooxidation on carbon paste electrode modified with the newly developed nanocomposite was investigated. Preliminary experiments showed that the nanocomposite had an electrocatalytic effect on the electrooxidation of STB and a significant improvement in the anodic current of the drug was obtained at + 0.66 V. Under the optimum conditions, the analytical figures of merit were extracted and the method was fully validated. A relatively wide dynamic linear range of 0.018-18.77 mu mol L-1 with the detection limit of 0.9 n mol L-1 was obtained. Furthermore, the method was applied for the determination of STB in biological fluid samples such as human serum samples with acceptable recoveries (97-102), satisfactory precision (2-4% RSD) and minimal matrix effects. (C) 2019 The Electrochemical Society.