A multipoint fiber-optic temperature sensor network integrated inside a power transformer for continuous monitoring of hot-spots on windings, cellulose insulations, and oil, is demon- strated and tested. The temperature sensors are based on propri- etary encapsulated fiber Bragg grating (FBG) sensors and the op- tical interrogation unit uses a special designed narrowband high power broadband fiber source. The fiber-optic sensing network is integrated into a 20MVA, 345/ kV kV power transformer (CORE type) having 12 temperature sensing points, distributed over several physical locations inside the transformer (windings, cellulose insulators, magnetic circuit, and cooling oil entrance and exit).