Inevitably, cable structure is often used as a structural part of either a middle or long-span bridge, so its existing tension force is essential to be evaluated and monitored periodically. This research aims to demonstrate the use of covariance type of Stochastic Subspace Identification (SSI-COV) to identify the dynamical system properties of bridge's cable and estimate the cable tension. The concept of effective vibration length proposed by previous researchers is implemented in this research to overcome the unknown rotational stiffness of the cable's supporting system. During the measurement, multiple-synchronized vibration measuring devices are deployed on a cable to get the corresponding eigenvector and its effective length. The cable force of an existing bridge is identified using two different approaches, the least-square approximation and the two-frequency method. It is found that the deviation between those two mentioned approaches is very small. The overall process evidence that this non-destructive testing procedure is practicable to estimate the internal force of a cable in real structures.