Abstract: Because of the merits of optical fiber sensor comparing to other sensors, it is especially adapted to monitor the functions of composites and its varying with surroundings when embedded into composites during production, this technology has developed with the study of smart materials and structures. This paper analyzed the relationship between the varying of optical phase in the fiber and strain induced into by load or change of surroundings of composites using the results of weak wave guide theory; the special case of single mode Corning optical fiber has shown the sensitivity of optical phase changing to strain; meanwhile experimental results were provided to prove the theory, and errors were also analyzed.