Here we report a highly sensitive and stable detection of $NAD^+$ and NADH by the electrode on which diaphorase (DI) is covalently immobilized in viologen polymer network. The network is prepared by the covalent formation of the structure by mixing propylamine viologen (PAV), poly(ethylene glycol)(400) diglycidyl ether (PEGDGE), an diaphorase (DI). The PAV/PEGDGE/DI modified electrode has the sensitivity of $0.02nA{cdot}{mu}M$ and the detection limit of $3{mu}M$ with a response time of 2 s ($t_{90%}$) for NADH sensing.