Fiber optic temperature sensors offer unique advantages when used to measure cutting temperature in machining processes. This paper presents novel miniature fiber optic temperature sensors and their application in cutting temperature measurement in Rotary Ultrasonic Machining (RUM) of titanium. The sensors were based on a Fabry-Pérot (FP) interferometeric principle. The endface of the fiber was wet etched. A piece of borosilicate glass was thermally deposited into the cavity on the etched endface to form an FP cavity. Temperature calibration experiments were performed. During RUM, the sensor was embedded in the titanium workpiece to monitor the temperature change using different ultrasonic power. The results demonstrate the advantages of fiber optic sensors including high accuracy and resolution, superior stability and repeatability, and good durability against harsh environment.