This paper starts with analysing the two special recording methods of X- ray holography, and considers the effect of the impulse response of recording material and the size of hologram on the resolution of X-ray hologram. The conclusion is that the smallest resolving distance of X-ray lensless Fourier transform hologram can not be smaller than radius of the focus spot of the X-ray condenser or the filter aperture used in recording, and the resolution of X-ray in-line hologram can not be higher than the cutoff frequency of recording material, when X-ray source is coherent.