Two problems in fine-grained superplasticity, whose details are indispensable to understand the nature of superplastic flow, are discussed using light metallic materials. First, the effect of temperature and grain size on superplastic flow was investigated for the inclusive understanding of the dominant diffusion process. The effective diffusion coefficient for superplastic flow involving the lattice diffusion coefficient and the grain boundary diffusion coefficient were precisely determined. It was found that the superplastic flow in pseudo-single phase aluminum and magnesium alloys is best described by a single constitutive equation using the effective diffusion coefficient. Second, the effect of intergranular and intragranular particles was examined. Two critical strain rates for the elimination of the effect of these particles were developed respectively by considering the suitable deformation models. The constitutive equation for superplastic flow under the influence of intragranular particles was proposed.