Currently, heat treatment is the most common and effective method for changing the properties of metals and their alloys. Ferrous alloys most often are heat treated and the most commonly used process of heat treatment is the quenching. At quenching the final structure and material's properties are formed during the fast cooling process. Therefore, the correct choice of cooling conditions is critical for quality of quenched parts. The simulation of quenching is one of the modern approaches for researching the cooling processes. In order to obtain reliable results, the correct input data for simulation are crucial. It is commonly accepted that the heat transfer coefficient (HTC) plays most significant influence on the simulation results. Meanwhile in the specialized literature there are quite different values of HTC even at equal cooling conditions and a few data regarding its influence on the simulation results. The aim of this paper is to determine to what extent the results obtained after simulation of quenching processes are influenced by HTC. Samples of different dimensions and ratios between length and diameter (L/D), made of medium carbon steel (C45 EN 10083), were studied. Results for the influence of HTC together with geometry of the quenched parts, especially samples with different ratio length/diameter (L/D), are presented in the article.