Abstract The cobalt hexacyanoferrate nanocrystals (Nano-CoHCF) with uniform shape and size were electrochemical deposited on the electrode surface modified with the overoxidized polypyrrole (OPyox) film in the presence of ethylene diamine tetraacetic acid (EDTA). The conceivable effect of EDTA and OPyox film was studied using SEM and electrochemical techniques. The experiment results showed that the nucleation and deposition rate of CoHCF on the electrode surface were controlled by the synergic action of EDTA and OPyox film, which resulted in the formation of uniform Nano-CoHCF. Moreover, some electrodeposition conditions for Nano-CoHCF film had been optimized such as pH, concentration of EDTA, as well as deposition cycles. The resulting modified electrode exhibited excellent catalytic activity and improved stability for the oxidation of hydrazine. Hydrodynamic amperometry was applied for the determination of hydrazine with a linear range of 2.0×10−7 to 1.1×10−5molL−1 and 1.1×10−5 to 4.1×10−4molL−1 with the maximum sensitivity of 2719.5μAmM−1cm−2.