Molecular structure and packing of various organopolysilanes with symmetric and asymmetric side-chains have been obtained by comparing the theoretical X-ray diffraction pattern with the experimental pattern. Molecular structure of the organopolysilanes with asymmetric side-chains, such as, poly( methyl propyl silane) and poly(methyl phenyl silane), is mainly determined by the intra-molecular van der Waals interaction between the adjacent side-chains, which is similar to that of the organopolysilanes with symmetric side-chains, such as, poly(di-methyl silane) and poly(di-propyl silane). Packing of the organopolysilanes is determined to occupy closely in space in both cases. However, density of the organopolysilanes with asymmetric side-chains is smaller than that of the organopolysilanes with symmetric side-chains because of the large space between the adjacent side-chains.