A brief review is presented on insitu measurements of formation temperature that has been conducted in seafloor boreholes drilled under the ocean drilling program (ODP) and its successor, integrated ocean drilling program (IODP). The best way to measure accurate formation temperatures in boreholes is to insert temperature sensors beyond the drillbit into the sediment layer that is not thermally disturbed by the drilling. One is to use hydraulic piston coring system, where a temperature sensor and data logging system is mounted in the cutting shoe part. The other is to use a temperature probe or tapered lance. Depending on the stiffness of formation, the former allows to obtain in-situ temperature down to approximately 100 m below sea floor, whereas the latter can penetrate to approximately 400 m below sea floor. The overall accuracy of the temperature is on the order of 0.1 K to 1 K, which is mainly restricted by the crack created in the formation and the frictional heat caused at the time of penetration into the sediment. So far measurements were made in various environments such as in the hydrothermal areas or around subduction zones, and some evidence for subseafloor fluid migration has been obtained.