The combination of fiber optics with nanostructure technologies offers great potential for the realization of novel sensor concepts. Miniature optical fiber sensors based on the Fabry-Perot (F-P) structure are presented. The transducer deposited on the fiber end face is multilayer coating consisting of a stack of nanoporous dielectric TiO2 and MgF2 films of optical thickness \lambda/4, forming a F-P filter with a typical reflection minimum at about 1300-nm wavelength. The reversible adsorption and desorption of water molecules in the porous films depend on water vapor, which would shift the reflected minimum wavelength of the F-P filter. Therefore, the humidity sensing is correlated with the central wavelength change of the F-P filter. A 14-nm shift in central wavelength is observed when the relative humidity increases from 0 to 100%. The central wavelength response to relative humidity is 0.9969 in linearity, which means that the proposed F-P thin-film sensor is very promising as a relative humidity sensor.