The paper presents the results of the researches regarding the determination of the thermal conductivity coefficient of the moulds used for cast iron parts in Romanian foundries. The instantaneous values of the thermal conductivity coefficient of the moulds are influenced by the type of materials that compose the moulding batch (sand, binder, additional materials) their content (percentage) their characteristics (grains form and dimensions), but also by the temperature. Many software used for casting solidification uses a mean substitutive value. This one include the effect of heat transmission by conduction in the mould wall and the secondary processes that influence the heat transfer throw the mould wall ( burning processes of organic substances, water evaporation and re-condensation processes, mass transport processes). The determination of this mean value in the case of casting grey cast iron parts with thickness of 20 mm is presented in the paper. A regressive method was applied. The solidification time experimentally determined throw thermal analyses is compared with the solidification time obtained by simulation, in three points of the casting. The value of the substitutive coefficient of thermal conductivity that assure the best closeness between the simulated solidification time and the solidification time experimentally determined throw thermal analysis in the three points was established.