In this study, a back calculation formula using frequencies of two arbitrary modes of vibration is proposed to compute the tension force in pre-stressed members. Derivation of the proposed formula is based on the vibration theory of an axially loaded flexural member. This paper describes the use of the proposed formula to successfully recover the tension forces of lab-controlled pre-stressing strands. Data of field tests confirm that the proposed formula is also capable of predicting cable forces of a cable-stay bridge in good agreement with those obtained from traditional first frequency calculation and those by in-situ instrumentation with errors within acceptable range. As a result, it is concluded that such calculation can also be practically useful in giving reference values for other main effective methods regardless that reliable values of the first frequency of lateral vibration is available or not.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.