This Paper Reports a Fiber In-Line Fabry–Perot Interferometer (FPI) Fabricated in a No-Core Fiber using the Direct Femtosecond Laser Writing Technique for High-Temperature Sensing Applications. Two In-Line Reflectors Are Directly Inscribed in a No-Core Fiber to Construct a Low-Finesse FPI. Fringe Visibility Greater Than 10 DB is Obtained from the Reflection Spectra of the Fabricated No-Core Fiber FPIs. Temperature Responses of a Prototype No-Core Fiber FPI Are Characterized Up to 1000 °C. the Proposed Configuration is Compact and Easy to Fabricate, Making It Attractive for Sensing Applications in High-Temperature Harsh Environments.