Netlike flow (NF) and homogeneous flow (HF) are two basic types of ductile solid-state flow. NF is a viscoplastic flow, i.e. a viscous flow accompanied by localized ductile shear deformation, forming plastic-flow belts and their conjugate network, i.e. plastic-flow network (PFN), which is dominant in the lower crust and mantle lid, playing an important role for controlling intracontinental tectonic deformation, stress field, seismicity, and other geological-geophysical processes. HF, or called convective flow (CF), is a viscous flow without solid deformation localization but similar to liquid-state flow with heterogeneity in thermal, density and velocity structures, which exists in the asthenosphere and deeper mantle. Based on a viscoplastic model in which NF and HF are taken into account, a “power / linear-binomial” combined flow law is proposed for describing the rheological behavior of steady-state creep. The netlike-flow coefficient, β, is used in it for measuring the development level of PFN in specimen and the conventional power flow law is only a special state of the combined flow law with β equal to zero. The combined flow law provides a theoretical basis for understanding the rheological behavior of the Earth's interior and confirms further the inevitability of PFN in some depth ranges of the crust and mantle.