A fiber optic voltage sensor is developed using the photoelastic effect in a single-mode optical fiber, excited with a 632.8-nm HeNe laser. The force due to the mechanical deformation of piezoelectric material induced by voltage produces the photoelastic effect in the optical fiber. It is found that the sensor output voltage is proportional to the applied voltage up to 300 V. The frequency of the output voltage is equal to that of the applied voltage. The use of a commercial single-mode fiber rather than a low-birefringence fiber gives an economic advantage. In addition, the sensor’s structure is designed to be installed easily on a coaxial cable and to reduce the temperature effect. Experimental results from a laboratory model show a temperature coefficient of 0.076%/K.