In this paper, based on equations of lateral vibrations of wires, a non contact method for measuring instantaneous tension in a wire is introduced. The lateral vibration of the string is measured using an eddy current transducer and is sent to a computer. This signal is then imported to suitable software for fast Fourier transform (FFT) processing. The first natural frequency of the string is calculated online using FFT plot of the imported signal. This proposed method can be used for measuring tension for both fixed and moving wire with constant velocity, which is a usual case in wire processes. The results show the good agreement with theoretical values and the errors of the method is up to 8% at the worst case. This experimental study shows that this simple and low cost new method can be included in existing non-contact wire measuring tension apparatuses.