Due to strict governmental regulations of high energy performance buildings, sufficient thermal characteristics of building’s envelope must be reached. This reflects in low U values of structural complexes. Thermal transmittance through the complex should be as low as possible. Heat transfer through the structural complex depends on the calculated U value. U value can be determined by onedimensional (1D) and two-dimensional (2D) calculation method described in European standards SIST EN ISO 6946 and SIST EN ISO 10211. In practice both methods are used, although the use of 1D method is much more frequent, as it is obligatory by regulations. In this graduation thesis the comparison of 1D and 2D method is presented. For the calculation of U value by 1D method computer software TEDI is used, while for the calculation of U value by 2D method computer software THERM is used. Both listed software were used for calculation of U value of seven different structural complexes. The comparison of 1D and 2D method is based on results obtained by the above mentioned tools. The results calculated by 1D method were different from results calculated by 2D method. In general results collected by 2D method presented larger U values. This shows that 1D method is not reliable.