The present work proposes a new single strain gage technique for the accurate determination of mode I stress intensity factor (SIF) in orthotropic composite materials. This work also aims at showing the existence of valid or optimal gage locations for the accurate determination of SIFs in orthotropic composites. Accordingly, a finite element based general approach is developed for the estimation of such optimal gage locations. Results of numerical examples show that the proposed strain gage technique can yield highly accurate value of SIF for orthotropic laminates when the gage is placed within the valid locations.