  On basis of the first principle calculation we show that a crystalline
structure of silicon, as a novel allotrope with nanotubular holes along two
perpendicular directions, is stable. The calculations on geometrical and
electronic properties reveal that this allotrope possesses a direct band gap
wider by 0.5eV than the indirect one of silicon with diamond structure. The
crystal belongs to I41/AMD space group, showing anisotropic optical properties
and Young modulus. The bulk modulus is 64.4GPa and the density is
1.9g/cm$^{3}$, lower than that of the diamond silicon due to the presence of
nanotubular holes. It is hopeful that the allotrope may widely expand
applications of silicon in many fields due to its direct band gap and specific
nanotubular structure.
