Failure of oil-filled-type (OFT) power transformers usually results in very big losses comprising the loss of the transformer itself as well as production loss in industry. Most of the energy lost in OFT transformers, induced by faults, will become heat at a location close to the fault, which then becomes a hot spot. The detection of hot spots in OFT transformers can be an effective way for transformer fault location and diagnosis. As the OFT transformer enclosure is completely sealed and is opaque, to date there is no effective way to find out hot spots in them. It is known that ultrasonic energy can be transmitted into the sealed transformer enclosure which is filled with the fluid, and ultrasonic waves can be affected by the temperature of the materials they propagate through; so the ultrasonic method can be a possible method for hot spot detection. In this paper, we describe a study on the possibility of developing a method to detect hot spots in an environment filled with a fluid, simulating a transformer. Trails using a different times-of-flight (TOFs) evaluation techniques and attenuation measurement of the ultrasonic energy propagated were carried out, which showed encouraging results for locating the hot spot as well as measuring the hot spot temperature in fluids; we also discuss the possibility of applying these results to the diagnosis of OFT transformers eventually. 