Gas-cooled Fast Reactor (GFR) is one of the generation IV reactor concepts. The fast neutron spectrum and high outlet temperature of the core make this reactor type a contributor to the sustainability of the nuclear technology. One of the main challenges of the concept is the development of new fuel assembly types, which can withstand the high temperature [1][2]. Because of the high temperature, the thermal conditions of the core have to be known in detail. The main issue of this paper is to investigate the corner subchannel of the ceramic core of the GFR demonstrator, ALLEGRO. First, models were built for the bare rod bundle to determine inlet boundary conditions for the further models. These models were also used to investigate the effect of the simplification of the geometry. The second step was to develop a model, which included active length ceramic rods with four spacer grids in the region. This model described the heat transfer in the active part of the rod bundle and the heat conduction in the clads. Main purpose of this model is to describe the thermal conditions in the region more accurately.