Ab initio molecular orbital calculations of the C-CCSi torsion and the [1,3] sigmatropic silyl shift in allylsilane (SiC3H8, 1) have been carried out. It is clarified that the skew conformer of 1 is more stable than other conformers. Its torsional energy depends on the sigma-pi hyperconjugation as compared with that of 1-butene (2). In the [1,3] silyl shift, the reaction coordinate of the silyl-group migration in the early and final stages is congruent with that of the C=CCSi torsion near the skew conformer. Divergence points on the inversion and retention paths are discussed from the viewpoint of the degree of nonplanarity of the migrating silyl group. The activation energy along the retention path is calculated to be lower than that along the inversion path at various levels of theory, which is in remarkable contrast to 2, in which the inversion path is more favorable. From the energetical viewpoint, the [1,3] shift of the silyl group is expected to proceed with retention of the silicon configuration at the migrating center. Although this looks like an exception to the Woodward-Hoffmann rules, this may be caused by the nearly degenerate HOMO and (HOMO-1) in the transition state on the inversion path in 1.