Melting and reduction of fine iron ore particles in the gas environment of a HIsarna smelting cyclone is a critically important topic, but very limited information is currently available except for some experimental data from high temperature drop tube furnace (HTDF). This work discusses the equilibrium state of reacting iron ore in the HTDF environment by thermodynamic calculations to strengthen the understanding of the HIsarna process. The limit of reduction termination of the ore particles was estimated in the calculation for the thermal decomposition and topochemical gas reduction. The theoretical calculation results are compared with the experimental data from the previous studies. Furthermore, variation of slag composition and iron valence states were estimated theoretically to understand the effects of post combustion ratio value and hydrogen/carbon ratio on the equilibrium state of the ore particles in the reducing gas.