In this Letter, we propose an in-fiber single-polarization diffraction grating based on a radiant tilted fiber grating (TFG), in which the s-polarization light could be diffracted from the fiber core to free space with the wavelength-dependent diffraction angle. For the first time, we have presented the theoretical model of angular dispersion of a radiant TFG by employing the Fourier optics analysis method. The angular dispersion of the radiant TFG has been numerically characterized in terms of tilt angle, period, and wavelength. In the experiment, we have measured the diffraction angles and angular dispersion of the radiant TFGs with tilt angles of 41°, 45°, and 47° UV-inscribed into single-mode fiber, where the experimental results matched well with the simulation results. The simulation and experimental results have indicated that the diffraction light of radiant TFGs has a linear polarization state with over 0.99 degree of polarization at the tilt angle range from 41° to 47°.