In this work, pure yttria moulds have been prepared for directional solidifications of Nb-silicide based alloys with a withdrawal rate of 1×10−4m·s−1 at 1900°C. The influence of Hf contents on the metal–mould interfaces and contaminations was evaluated and the interaction mechanisms were described. In the alloys contained Hf, a continuous reaction layer consisting of HfO2 and Y2O3 was formed at metal–mould interface, causing some inclusions dispersed in the metal matrix. However, in the alloy without Hf, no apparent reaction layer but a segregation of Ti was found at metal–mould interface, and there was no visible inclusion in the alloy. All alloys were slightly contaminated with oxygen and yttrium as the result of slow thermal dissociation Y2O3(s)→2YO (g)+O (g). However, with the decrement of Hf contents in Nb-silicide based alloys, the metal–mould interaction weakened, resulting in the reduction of contaminations with oxygen.