High-resolution synchrotron radiation diffraction results indicate that the room temperature tetragonal (T) phase in (93-92) %Pb (Zn1/3/ Nb 2/3) O3 - (7-8) % PbTiO3 [PZN-(7-8)%PT] single crystals is a metastable phase stabilized by the transformation-cum-cooling stress in the crystal. This is manifested by the "missing" (001) T diffractions when the diffractions were taken from (001) fractured surfaces. The stress relaxation in this case has caused the metastable (001) T domains in the surface layer to transform to the equilibrium rhombohedral phase. Temperature and particle size effects on the metastable domains have been examined and discussed. Based on the present finding, a new PZN-PT phase diagram showing extended rhombohedral+tetragonal region is proposed.