Mathematical models of glass tank furnaces are formulated under the assumptions of two-dimensional and steady-state condition in the central longitudinal cross-section of the tank furnaces. Models are mainly described by 1) equations of mass, momentum and energy continuity for molten glass, 2) equations of heat transfer by thermal radiation among combustion flame, crown refractory and molten glass or batch surface and 3) equations of heat flow by thermal conduction through walls. Most equations subsequently undergo a well-known transformation into difference forms for the computer relaxation method.These models are applied to calculations of 1) temperature and velocity distributions in molten glass with surface temperature profile given as a boundary condition, 2) effects of furnace shapes (step-up, slope-up, throat, etc.), 3) effects of physical characteristics, 4) temperature distribution of glass ribbon in a tin bath and 5) relations of temperature and velocity distribution of glass and consumption of oil or batch feed temperature. Results of the simulation are adopted, in part, for tank design and are thought to be useful for basic understanding of controlling glass tank furnaces.