Light monosilane SiH4 belongs to a large group of molecular substances consisting of tetrahedral molecules. In the solid state most of them experience structural phase transitions accompanied by changes in the orientational order and symmetry of crystals. In solid SiH4 the phase transformation occurs at Tc = 63.45 K. The previous spectroscopic and structural data do not permit unambiguous identification of the crystalline structure of the SiH4 phases.  To solve this problem, this X-ray study was carried out in an interval of 6–80 K using diffraction and photometric methods. It is found that both the SiH4 phases have monoclinic lattices. The phase transition entails a marked jump of the volume ΔV=1.8 %. On changing into the high-temperature phase the extent of the orientation disorder increases, which manifests itself in the increasing number (from 4 to 32) of molecules in the monoclinic cells of the phases.  Classification of the symmetry of crystals is performed using the symmetry elements that correspond to the structure: rotation axes, screw axes, inversion axes, symmetry planes, glide planes. To determine the symmetry group of the monoclinic structure, we find symmetry elements that correspond to its structure.  It is determined that the silane has a primitive lattice, which which does not have glide planes and screw axes. Consequently, it can be stated that two symmetry groups can correspond to the low- temperature of a monosilane: P2 or P2/m.