The numerical simulation of time evolution of thermal and mechanical fields in a selected section of a massive concrete dam was undertaken by means of the Finite Element Method (FEM) using 3D modelling. Low- (i.e. with time period – year) and high frequency (i.e. with time period – day) cycles of environment temperature variation were considered. The results of the computation confirmed the hypothesis of the vulnerability of the structure to thermal influences and explained the mechanism of the structural defects observed.