We describe an ultrasensitive electrochemical detection of DNA protocol based on the isothermal strand-displacement polymerase reaction (ISDPR) and enzyme dual amplifications. Target DNA triggered an ISDPR to produce numerous bi-functionalized duplex DNA complexes. Following an immuno-magnetic collection via an immunoreaction between the attached digoxin on the duplex DNA and the anti-digoxin antibody on the magnetic bead, horseradish (HRP) tracers were bound to the duplex DNA through a biotin–streptavidin interaction. The quantification of DNA was realized by square wave voltammetric detection of the enzymatic products with a screen-printed gold electrode. The voltammetric response was proportional to the concentration of DNA in the range of 0.1 fM–0.5 pM, and the limit of detection was estimated to be 0.06 fM. The new protocol showed great promise for simple, cost-effective, and quantitative gene analysis. Keywords: Electrochemical, DNA, Strand-displacement, Enzyme, Amplification