Abstract: In this paper, a novel electrochemical sensor was fabricated based on nitrogen-doped reduced graphene oxide (N-RGO). The morphology, structure and electrochemical properties of N-RGO were investigated by transmission electron microscopy (TEM), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). AFM indicated the thickness of N-RGO is about 1.0 nm. The electrochemical experiments demonstrated that N-RGO possesses a relatively large surface area, strong adsorptive ability and excellent electrical conductivity. It was believed that N-RGO is a promising candidate for applications in electrochemical sensors and biosensors. Moreover, N-RGO modified glassy carbon electrode (N-RGO/GCE) exhibited good electrochemical response toward the oxidation of guanine with a linear range covering 4.14 × 10−7–3.71 × 10−4 M, and the corresponding detection limit (LOD) of 1.38 × 10−7 M. Eventually, the proposed sensor could be used to monitor cell necrosis by means of detecting the increase of the current response of guanine. Thus, the work is very meaningful in the field of biological cytology and pathology. A sensitive sensor for guanine detection is constructed based on nitrogen-doped reduced graphene oxide (N-RGO), which was synthesized by a facile method. This paper is the first time to monitor cell necrosis by detecting the increase of guanine content.