Abstract: Based on high resolution experimental X-ray powder diffraction spectra of SRM-2 molecular sieve, the patterns of its polycrystalline structure were indexed, analyzed and simulated by molecular structure analysis software, which provided important information on its crystal structure, such as space group, and crystal lattice parameters. From the known crystal structure parameters, an approximate structure was first assembled and refined with Rietveld method subsequently. Results show that in SRM-2 molecular sieve, α cage is consisted of 48 framework atoms (twelve 4-membered rings, eight 6-membered rings and six 8-membered rings) and α cages are connected by double 8-membered rings with pore diameter of 0.419 nm. The crystal structure of SRM-2 molecular sieve belongs to cubic crystal system, 123 space groups. Its unit cell parameters are a = b = c = 1.511 1 nm，∠ =∠ =∠ = 90.00°.

Key words: X-ray powder diffraction, structure determination, Rietveld refinement