The NiO/YSZ interface prepared by depositing NiO on a single crystal YSZ (111) substrate has been investigated by transmission electron microscopy. As deposited, a very thin nickel layer ascribing to the non-stoichiometry at the very beginning growth of NiO and an amorphous silica phase resulting from silicon segregation were present at the interface. The orientational relationship of NiO (1 (1) over bar1)//Ni (1 (1) over bar1)//YSZ (1 (1) over bar1) with NiO [110]//Ni [110]//YSZ [110] was observed. The microstructural and chemical changes at the NiO/YSZ interface after being heated in vacuum and hydrogen indicated different reduction mechanisms. In vacuum, the reaction NiO -> Ni + 1/2 O-2(g) was prevailing at the interface between NiO and pre-existing Ni, which led to the thickening of nickel layer. In hydrogen, the reduction initiated on the NiO surface was dominant, following the chemical equation H-2 + O-O (NiO) -> H2O (g) + V-O(..) (NiO) + 2e (Ni).