MicroRNAs (miRNAs), a type of non-protein-coding and small RNAs, are encouraging molecular biomarkers for prior diagnosis of the certain diseases. Here, an electrochemical sensor based on iron oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate (Fe3O4/PEDOT:PSS) hybrid coated glassy carbon electrode (GCE) is reported to determine microRNA-21 (miRNA-21) as a biosensing platform based on hybridization event. The biosensor was characterized by cyclic voltammetry (CV) measurements. The Fe3O4/PEDOT:PSS demonstrated good sensing performance, including a wide detection range (0.1 pM-1 µM) and a low detection limit (0.023 pM) for potential medical applications.