The packing of poly(di-n-alkylsilylenemethylene) (PDASMs) chains was studied by using X-ray, electron diffraction, and molecular modeling methods. X-ray and electron diffraction measurements revealed unit cells in which the PDASMs were efficiently packed. The PDASM with the longer alkyl side chains, such as poly(di-n-propylsilyenemethylene). (PDSM), showed packing with the alkyl side chains interlocked with each other like cross-shaped gears in the two-dimensional monoclinic unit cell. The PDASM with the shorter ethyl substituent, poly(di-n-ethylsilylenemethylene) (PDESM), showed a lack of ability to interlock its side chains due to the short length of the alkyl groups. In these studies, we found that the length of the alkyl side chains could change the packing arrangement of PDASMs from monoclinic to orthorhombic to hexagonal with only short-range order as the alkyl side chain length decreases at room temperature.