The orientation relations m(100) ∥ t(001), m[001] ∥ t[110]; m(011) ∥ t(100), m[100] ∥ t[001]; m(100) ∥ t(110), m[001] ∥ t[001]; m(013) ∥ t(116), m[001] ∥ t[001] (indices for the primitive tetragonal cell) have been found between the tetragonal (t) and monoclinic (m) domains during the electron irradiation-induced m-t phase transition observed in-situ with HREM within isolated zirconia nanoparticles. Geometric models of the m-t interfaces are proposed.