Fiber Bragg grating (FBG) sensors are usually used for the measurement of axial strain. However, when a non-axisymmetric transverse strain is applied to the FBG sensor, the reflection spectrum from the sensor splits into two peaks because of the birefringence effect. Using this phenomenon, in this research, the FBG sensor was applied to the measurement of transverse thermal residual strain in CFRP laminates. First, the FBG sensor was compressed transversely in order to investigate the deformation of the reflection spectrum due to the birefringence effect. The wave-length shifts of the two peaks in the spectrum agreed with those calculated theoretically. Then, the FBG sensor was embedded into a CFRP cross-ply laminate, and the wavelength shifts of the two peaks were measured as the temperature of the laminate was changed. From the peak wavelengths, the non-axisymmetric component of the thermal residual strain in the laminate could be obtained in consideration of the birefringence effect and the strain transfer from the laminate to the optical fiber. Furthermore, the effect of the fiber coating on the measurement sensitivity was clarified theoretically.