Fiber Bragg grating (FBG) sensors have been applied to the detection of transverse cracks in carbon fiber reinforced plastic (CFRP) cross-ply laminates using the sensitivity of the sensors to non-uniform strain distribution. However, there is the possibility that the resin coatings of the FBG sensors will relax the non-uniform strain distribution and deteriorate the sensitivity of the sensors. Hence, in the present research, the strain transfer from the laminate to the core of an embedded optical fiber was calculated by a theory developed from the shear-lag model. As a result, it was found that the variation of the axial strain in the core of a polyimide coated FBG sensor was smaller than that of an uncoated FBG sensor. Nevertheless, the reflection spectra of the coated FBG sensor and the uncoated one, which were calculated from the strain distributions in their cores, were almost the same. This result shows that the coating of the optical fiber does not deteriorate the sensitivity of the FBG sensor for crack detection. Furthermore, the effectiveness of the coated sensor was also confirmed experimentally.