The characteristics of Kα X-ray sources generated by p-polarized femtosecond laser-solid interactions are experimentally studied in the relativistic regime. By use of knife-edge image technique and a single-photon-counting X-ray CCD camera, we obtaine the source size, the spectrum and the conversion efficiency of the Kα X-ray sources. The experimental results show that the conversion efficiency of Kα photons reaches an optimum value of 7.08×10-6/sr at the laser intensity of 1.6×1018W/cm2, which is different from the Reich's simulation results (Reich et al., 2000 Phys. Rev. Lett. 84 4846). We find that about 10% of laser energy is converted into the forward hot electrons at the laser intensity of 1.6×1018W/cm2.