Effects of redox cycling on the mechanical properties of Ni-YSZ cermets were investigated by using an in-situ small punch (SP) testing method. The Ni contents in the Ni-YSZ cermets tested in this study were 0, 15, 30 and 50 vol.%. Electrical conductivity measurements suggested that Ni particles formed a network-like structure at Ni contents greater than 20 vol.%. Experimental results obtained from the in-situ SP tests at 800°C demonstrated that ductile-like deformation was induced under reducing environment (at an oxygen partial pressure of 10 -18 atm) for the Ni contents of 30 and 50 vol.%. The ductile-like behavior was attributable to the formation of Ni network. The first complete re-oxidation of Ni-YSZ cermets then caused critical damage and resulted in drastic strength reduction, probably due to Ni agglomeration. This study revealed the first evidence for the presence of potential plastic deformation capacity in Ni-YSZ cermets under reducing environments.