The X-ray structure analysis of isotactic poly(methyl methacrylate) was performed by adopting the 10/1 double-stranded helix model as the molecular model, and the crystal structure analysis, which has remained as an unsettled problem for a long time, has been brought to an end. The crystals are orthorhombic, with rather large cell dimensions, i.e. a=41.96, b=24.34, and c (fibre axis)=10.50 angstrom. There are eight 10/1 double-stranded helices of the right-handed and left-handed senses in the unit cell, which take up a slightly complicated hexagonal-like closest packing-mode, and where among the first six neighbour helices the two same-handed senses are located at z = c/2 and the four opposite-handed senses are located at z = c/4 and 3c/4. The space group is Fddd-D2h24 for the disordered structure, which is composed of upward and downward helices related by a twofold rotation axis perpendicular to the helix axis. This complicated helical packing-mode originates from (i) the fact that iPMMA as synthesized is an apparently racemic polymer, and (ii) the fact that two molecular chains form a robust and rod-like 10/1 double-stranded helix.