In the present study, wholly of electrochemical methodologywas used to acquire an inexpensive and stable graphene oxide/cobalt oxide nanocomposite on a pencil graphite electrode (PGE). At first, the graphene oxide (GO) nanosheets were directly synthesized at pencil graphite electrode as a carbon source via a potentiostatic method in sulfuric acid solution. Then, cobalt oxide nanoparticles (CoO(x)NPs) was loaded by cyclic voltammetry on the GO-coated PGE. The morphology of unmodified PGE, GO/PGE and CoOxNP/GO/PGE were characterized by scanning electron microscopy (SEM). The catalytic properties of the quickly designed sensor were used to appraise the electrochemical behavior of insulin. The electroactivity of insulin was significantly enhanced on the CoO(x)NPs/GO nanocomposite compared with unmodified electrode. A linear dynamic range between 0.46 to 100 nmol dm(-3) were obtained with a detection limit of 0.12 nmol dm(-3) and a superior detection sensitivity 0.687 mu A/nmol dm(-3). Also, the sensor response was not damaged by the presence of common biological intruders such as ascorbic acid, uric acid, citric acid, and glucose. Eventually, three pharmaceutical insulin samples from three different brands (Regular, Isophane, and Lansolin) were selected and analyzed. The recovery percentage suggests that the proposed sensor could be utilized in routine analysis of pharmaceutical preparations. (C) 2019 The Electrochemical Society.