The paper deals with phase diagram calculations and predictions of selected binary, ternary, and Zr-Nb-O-H quaternary system. These systems are important as alloy base for cladding materials involving fissionable fuel in nuclear industry. The main attention is focused on the phase diagram prediction of Zr-1wt.%Nb-O alloy system at temperature conditions up to 2 000 °C. In addition, the influence of hydrogen content on the Zr-1wt.%Nb-O phase diagram is discussed. According to service conditions in nuclear industry, 1 000, 2 000 and 3 000 ppm of hydrogen contents are considered. The achieved results are presented as iso-concentration section of the phase diagrams. Impact on mechanical properties of the material under view is discussed.