The pore shape and size of MCM-41 were studied analytically by comparing the observed powder X-ray diffraction intensities with that derived from the MCM-41 crystal structure models, with two different pore shapes, a hexagon and a circle. The powder diffraction patterns from the as-synthesized and the calcined MCM-41 were measured by a synchrotron radiation at SPring-8, Japan. The MCM-41 structure with circular and hexagonal pore shapes explains well for the as-synthesized and the calcined MCM-41 crystals, respectively. The pore size and boundary obtained by this approach agree with those obtained from an N-2 gas adsorption measurement combined with the Fourier synthesized density map.