The microwave spectra of disilylmethane and its isotopically substituted species were measured. From the observed rotational constants, the rs structural parameters were obtained by the substitution method. Form the Stark effect measurements of low J transitions, the dipole moment of disilylmethane was found to be 0.819 ± 0.002 D. From the fine structure in the ground state rotational spectrum, the barrier to internal rotation of the SiH 3 group was obtained to be 1210 ± 40 cal/mole, neglecting the coupling effect of the two silyl groups. The barrier was also obtained for several isotopically substituted species. Discussions are given on the structure, dipole moment and barrier to internal rotation of the silyl group for disilylmethane in comparison with those of similar molecules.