In this paper, investigation of the effect of packaging material on optical fiber temperature sensor with single-mode multimode single-mode (SMS) structure has been done. First, the optimum length of multimode fiber that will be used as temperature sensor is chosen. The tests of SMS temperature sensor have done by using simple measurement system of intensity. The results show that the 42.63 mm length of step-index multimode fiber has a better performance than length 44.12 mm based on theoretical and experiment. After that, SMS optical fiber packaged with material. The materials of packaging are alumunium and stainless steel. The results show that after using packaging, the sensitivity of sensor has been inmproved. It is caused by the thermal expansion coefficient of packaging material. The highest sensitivity is obtained by using alumunium packaging. The sensitivity bare SMS optical fiber is 0.033 dBm/°C by theoretical and 0.066 dBm/°C by experiment. The sensitivity of SMS optical fiber packaged improve to 0.073 dBm/°C by theoretical and 0.176 dBm/°C by experiment.