High-sensitivity temperature sensors have been used to validate real-time thermal responses in tissue during photothermal treatment. The objective of the current study was to evaluate the feasible application of a fiber Bragg grating (FBG) sensor for diffuser-assisted laser-induced interstitial thermotherapy (LITT) particularly to treat tubular tissue disease. A core-diameter diffuser was employed to deliver 980-nm laser light for coagulation treatment. Both a thermocouple and a FBG were comparatively tested to evaluate temperature measurements in ex vivo liver tissue. The degree of tissue denaturation was estimated as a function of irradiation times and quantitatively compared with light distribution as well as temperature development. At the closer distance to a heat source, the thermocouple measured up to 41% higher maximum temperature than the FBG sensor did after 120-s irradiation (i.e., for FBG versus for thermocouple; ). Ex vivo porcine urethra tests confirmed the real-time temperature measurements of the FBG sensor as well as consistently circumferential tissue denaturation after 72-s irradiation (). The implementation of FBG can be a feasible sensing technique to instantaneously monitor the temperature developments during diffuser-assisted LITT for treatment of tubular tissue structure.