Abstract: he method of measuring the focal spot size of X-ray source was designed by using a monolithic glass polycapillary X-ray collimator (MGPXRC). The MGPXRC is composed of 10⁵ capillaries with an inner diameter of several microns. The divergent X-ray beam emitted from X-ray source became a quasi-parallel beam after collimated by MGPXRC. The quasi-parallel beam size is related to the focal spot size of X-ray source. Therefore, the focal spot size of X-ray source can be obtained by using the linear fit based on the quasi-parallel beam size at various distance from the output of the MGPXRC. The focal spot size of a micro X-ray source was measured by using the MGPXRC method and conventional pinhole imaging method, and the measured results are 60.8 and 59.4 μm, respectively. The MGPXRC can be used to measure the focal spot size of X-ray source with a larger emitting angle than the critical angle of the total reflection of X-rays. And the MGPXRC method has a potential application in the measurement of X-ray source with a spot size of a submicron or even smaller.