An original three-dimensional (3D) model of solidification is used to describe the process of solidification and cooling of massive 500x1000x500 mm cast-iron castings in sand moulds. The calculated model of the kinetics of the temperature field of the casting is verified during casting with temperature measurements in selected points. The following dependences are later determined according to the experimental and calculated data: the average size of the graphite spheroids rg, graphite cells Rb and the average distances among the particles of graphite Lg – always as a function of the local solidification time θ [xi, yi, zi]. Furthermore, it has been found that the given basic characteristics of the structure of the cast-iron (rg, Rb and Lg) are a linear function of the logarithm of the local solidification time θ. The original spatial model of solidification can therefore be used in its first approximation for the assessment of the pouring structure of massive cast-iron castings.