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A few-mode polymer optical fiber (POF) with a highly photosensitive core doped with benzil dimethyl ketal (BDK) was fabricated and used to write a Bragg grating using the modified Sagnac optical interference technique. Bragg gratings formed in the few-mode POF show multiple reflection peaks. The response characteristics of the temperature and the strain of the few-mode POF Bragg grating (POFBG) were theoretically analyzed and experimentally verified. The results show almost the same strain response but a different temperature response for all guided modes in the few-mode POFBG. A new method of measuring temperature and strain simultaneously was proposed and demonstrated experimentally by using a single few-mode POFBG.