We discuss mechanical stress sensors based on fiber Bragg gratings (FBGs) micromachined by anisotropic reactive ion etching (RIE) using CF4 plasma. Locally thinned FBGs have the potential to be used as sensitive strain sensors whose sensitivities increase with decreasing measurement range. The center reflection wavelength shift rate of an etched and thinned grating (thinned grating length: 2.0 mm; thinned diameter: 15 μm; total grating length: 10.0 mm) was approximately 15 times greater than that of a conventional unprocessed FBG. The difference in the center reflection wavelengths of thinned and unetched grating regions could be used to counteract changes in the surrounding temperature. On the other hand, a single FBG with an asymmetrical cross section can detect the degree of curvature and the bending direction. The center reflection wavelength shifted by 0.4 nm at 1550 nm on bending that imparted a radius of curvature of ±127 mm.