Some main factors having influence on the resolution of X-ray holograms are analyzed theoretically. The smalles resolving distance of X-ray lensless Fourier transform holograms can not be less than the 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 holograms can not be higher than the cutoff frequency of recording material, under the condition that X-ray is perfectly coherent. Relationships between the coherent length required by X-ray lensless Fourier transform holography and the resolution, and the radius of the object are quadric; and the coherent length requested by X-ray in-line holography is proportional to the square of the resolution and does not change with the size of object, under the condition that X-ray is partially coherent. With the resolution of X-ray holograms raised, the reguired coherent length will be increased by a large amount.