In this paper a uniaxial stress-strain formula of structural steel at constant temperature between room temperature and 600°C is presented, and also the thermal deformation analysis of 1 story 1 bay steel frames which uses the proposed stress-strain formula and the mechanical model of structural steel at high temperature proposed in Ref. 13) is carried out. The stress-strain formula consists of two straight line expressions which represent elastic part and plastic plateau and one curved line expression (l) which represents strain hardening part or stress-strain curves at high temperature and gives stress explicitly in terms of strain. The coefficients and the parameters in the formula are functions of temperature and those values are decided by experimental date which were presented in Ref. 6)〜 12). The numerical frame analysis method is one-dimensional finite element elasto-plastic creep analysis similar to that was used in Ref. 2), including thermal and the proposed mechanical properties of steel and geometrical nonlinearities of steel members. Two example frames are analyzed, one is desgined for both seismic load and gravity load, and the other is designed for gravity load only. The former calculation is continued throughout heating process and cooling process, the later frame collapsed at about 557℃ on heating process.