Based on the theory of semiconductor laser pattern and the non-paraxial vectorial moment theory of light beam propagation, the beam quality factor M^(2) of TE_(0) propagating mode is analyzed and calculated. The result shows that when both core layer and cladding layer are considered, M^(2) > 1 is always obtained. Moreover, by analyzing the characteristic of real beams, this result is generalized to the multilayer isotropic linear slab waveguides.