The paper presents the results of investigations concerning the formation and high temperature oxidation behavior of silico-boronized coatings on C103 Nb-base alloy. The coatings were obtained by boronizing and subsequent siliconizing using pack cementation method which resulted in the formation of inner NbB2 and outer NbSi2 layers. The coated C103 alloy was subject to isothermal oxidation test at 1100 °C for 1000 h during which the coating provided protection against high temperature oxidation of the base material. The oxidation behavior was studied using SEM and STEM methods and the samples from the metal-scale interface were prepared by Focused Ion Beam method. It was shown that during high temperature exposure the reaction between the NbSi2 and NbB2 layers results in the formation of Nb5Si3 and T2 phases. The oxidation process is hindered at the front of this reaction where mixed oxide scale was found consisting of Nb2O5, NbO2 and borosilicate glass.