Using X-ray and DAC, room-temperature crystal structures under physical pressure(PP) were solved for DCNQI-Cu compounds which exhibit particular metal-insulator(M-I) transition owing to their electronic low dimensionality(CDW) and strong correlation(Mott). This M-I transition is known to be triggered with moderate(=small) PP or chemical substitution(chemical pressure:CP). Observed high pressure structures were compared with theoretical structure determinations. From the structural viewpoint, relation between PP and CP effects on the M-I transition was discussed, [organic conductor, metal-insulator transition, charge density wave, Mott transition, X-ray diffraction]. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.