Purpose: This paper presents the results of the numerical calculations concerning the influence of the placing of the developing gaseous gap in a mould based on the thickness of the forming shell, and the temperature distribution on the strand length. A cast slab with dimensions of 1100 mm x 220 mm was analysed. The calculations were performed on various heights of the formation of the gap in the mould. Other process parameters, i.e. the casting speed, melt temperature, and the cooling intensity in the secondary cooling zone, were maintained at a constant level. Design/methodology/approach: The numerical model of the steel continuous casting process, developed with the ProCAST software was used. In the study the effect of the height of the air gap development was examined for five variants. In the heat transfer in the gap model, two basic heat transfer mechanisms were assumed: by radiation and by conductivity. Findings: The numerical model of the steel continuous casting process with influence of the placing of the developing gaseous gap in a mould was developed. The verification of the calculation results obtained, after conducting a number or mathematical simulations, concerned the shell thickness and its dependence on the height of the air gap. The simulation of the temperature distribution was made for the whole strand. Research limitations/implications: The numerical model of the steel continuous casting process, with the curved mould should be used to determine the influence of the placing of the developing gaseous gap. Practical implications: The results of the numerical calculations with heat transfer model, concerning the influence of the placing of the developing gaseous gap in a mould based on the thickness of the forming shell, and the temperature distribution on the strand length. Orignality/value: The calculations were performed on various heights of the formation of the gap in the mould using ProCAST software. The calculated temperature distribution was verified on the basis of an industrial database.