Once a geothermal site has been selected for exploration, a full understanding of the formation temperature distribution on the surface and in the underground reservoir is always needed to assess the geothermal capability and productivity. Commercially available electronic measuring instruments cannot overcome the highest downhole temperatures obtained in the world's deepest and hottest geothermal reservoirs. In this study, fiber optic Bragg grating (FBG) measurement technology is utilized applied in an attempt to replace more expensive electronic sensors and to obtain more accurate downhole pressure and temperature data. This has been our target since 2013. For this purpose, a portable pressure and temperature (P/T) optic fiber wire-line instrument prototype was developed, and it can measure up to 1500 m in depth. The prototype was named SINOT-ECH-OPTIC-P/T in 2015. It has been tested in various well types, including a hot spring well and a pioneering geothermal exploration well (JY-01). The primary results have proved to be satisfactory. This paper summarizes the temperature and pressure measurement results obtained in JY-01 well at Sanshing geothermal site in I-lan County, Taiwan. The maximum measured well depth was 1200 m, and data measured includes a downhole temperature profile with the maximum temperature of around 75 degrees C.