In this work，a label-free electrochemical sensor for microRNA－21 detection was reported，based on target-mediated self-assembly of DNA nanostructure and catalytic signal amplification．A thiol-tethered and stem-loop structured DNA capture probe（CP） and two sequence-specific DNA strands that were partly complementary to the CP and the target（microRNA－21） were used．Initially，the CP formed self-assembly monolayer on the surface of gold electrode vis Au－S chemistry，which further evolved to a H-shaped DNA－RNA complex in the presence of the target．The electroactive ions－［Ru（NH,3,）,6,］,3+, （RuHex） electrostatically adsorbed to the negatively charged phosphate moieties in the nucleic acid skeleton could generate strong electrolytic current through DNA-mediated electron transfer mechanism．By contrast，the current signal was rather weak in the absence of the target，due to the inexistence of the DNA－RNA complex．Furthermore，another electroactive specie，［Fe（CN）,6,］,3-, ions were ultilized to chemically oxidize the electroreduction product－［Ru（NH,3,）,6,］,2+, ions，thereby establishing an electrochemistry－chemistry cascade reaction and consequently realizing current signal amplification．Electrochemical impedance spectroscopy（EIS） was used to confirm the as-assembled DNA－RNA complex，and chronocoulometry technique was employed to calculate the capture probe density that may affect the electrochemical response．In addition，capture probe concentration，ratio and self-assembly time were optimized．The peak currents measured from differential pulse voltammetry（DPV） were proportional to the target concentration in the range of 0.1 fmol/L－0.1 nmol/L，with a detection limit down to 12.8 amol/L．Particularly，it could effectively distinguish other microRNAs and even single-base mismatch analogs，enabling to the detection of microRNA－21 in diverse cell lines．With the advantages of high sensitivity，favorable specificity and broad linearity，the reported sensor could work without the need of electrochemical probe labeling，and circumvents currently prevailing strategies such as PCR，rolling circle amplification and strand displacement reaction that are typically time-consuming and labor-intensive，sthus simplifying the analytical procedures and improving the practicality.