The popularly accepted concept of "stress induced phase transformation (SIPT)" for tetragonal zirconia polycrystalline (TZP) ceramics has been re-evaluated in this work using an in-situ X-ray diffraction technique-that was facilitated by the use of a novel stressing fixture. At stress levels of 700 MPa, which is close to the sample’s rupture strength very little of the tetragonal phase transformed to a monoclinic phase, regardless of whether a tensile or compressive stress was applied. However the intensity of the peak (202)(t), (220)(t), (113)(t), and (131)(t), compared with the peak, (111)(t) did display a significant change after the tetragonal zirconia was loaded. In the fractured surface, a large amount of monoclinic phase was discovered. Thus we infer that for a homogenous TZP ceramic, the critical phase transformation stress is close to the material’s rupture strength. On the basis of the observation of a non-linear deformation before the phase transformation, we suggest that the TZP material may have a four step response to an increasing applied stress. This response consists of; (i) anelastic behaviour which may be explained by "ferroelastic domain switching" or another anelasticity theory; (ii) t --> m phase transformation; (iii) microcracks emerging and then growing; (iv) final fracture of the material and a possible reverse transformation.