We investigated a packaging scheme for a singlemode-multimode-singlemode (SMS) optical fiber structure for a temperature sensor. The packaging material functions to protect the small and fragile sensor against the harsh environment as well as to increase the sensor sensitivity. Polytetrafluoroethylene (PTFE) material was chosen due to its excellent dielectric properties; hence the sensor applies to areas with electromagnetic interference. The effect of PTFE packaging is observed numerically and experimentally on a 43-mm multimode fiber-length SMS fiber sensor at the temperature range of 25–100℃. The temperature change causes thermal expansion of optical fiber and the packaging, therefore, induces changes in the optical power output of the sensor. The numerical calculation is based on modal propagation analysis of the light in the SMS fiber structure, while taken into consideration the coefficient of thermal expansion (CTE) of packaging material. The experiment includes the packaging in a rectangular shape, with 1, 2, 3, 5, 6, and 10-mm PTFE thickness, applied in single-sided design. Intensity-based interrogation method shows that the sensitivity of the sensor increases when PTFE packaging is applied, compared to the bare SMS fiber structure sensor.