The strain measurement by the fiber Bragg grating (FBG) is achieved by the change of FBG′s wavelength upon axial strain. But in the complicated stress state, the FBG′s strain respond error occurs due to the transverse effect. The impact of direction of FBG on strain measurement is proved through theoretical analysis and experimental results. The relationship between direction of FBG and strain measurement error in the state of uniaxial stress was derived and tested. The results indicate that the error increases rapidly when the FBG angle is about 60°, the results of strain are distorted extremely. In order to remove the error, modified formulas for strain were derived.