A new fiber in-line Mach–Zehnder interferometer based on a straight waveguide along the central axis of the no-core fiber sandwiched between single mode fibers is fabricated by a femtosecond laser. The device can be used for high temperature sensing with a sensitivity of −278.86 pm/°C and for bending sensing with a sensitivity of 0.28 nm/m−1. The high mechanical strength, simple fabrication, and precisely controlled free spectral range make the device attractive for potential applications in high temperature monitoring.