In this study. a technique using wireless impedance sensor nodes is proposed to monitor tensile force of structural cable. In order to achieve the goal, the following approaches are implemented. First, a wireless impedance sensor node is designed for automated and cost-efficient prestress-loss monitoring. Second, a tensile force monitoring technique using an interface washer for structural cables is proposed to overcome limitations of the wireless impedance sensor node such as measurable frequency ranges with narrow band. Finally, the applicability of the wireless impedance sensor node and the technique using the interface washer is evaluated in a lab-scaled prestressed concrete (PSC) girder model.