We report a low loss photonic crystal slab waveguide formed by deforming the innermost holes on either side of a single line defect so that the circular air holes are changed to elliptical holes. The group velocity and group velocity dispersion of this waveguide depend strongly on the innermost elliptical air holes. We obtain the photonic bands and group index of guided modes in this kind of photonic crystal waveguide by guided mode expansion method and investigate the dependence of photonic bands and group index of guided modes in this photonic crystal waveguide on the parameter of the innermost elliptical air holes. For waveguides with the optimum innermost elliptical air holes, we achieved a wider single mode region below the light line, in which light can easily propagate without intrinsic loss. At the same time, this kind of waveguide has nearly constant group velocity and vanishing group velocity dispersion in a 3—5 nm bandwidth. These results will be applicable to design and fabricate photonic crystal slab waveguides with low group velocity, low dispersion and low loss characteristics.