Single crystals of SrGdMnO 4 have been synthesized by the floating zone method. The structure was characterized as the K 2 NiF 4 -type, using X-ray diffraction (XRD) and a transmission electron microscope (TEM). Presence of weak reflections breaking the archetypal tetragonal symmetry was observed from the selected area diffraction (SAD). The compound was found to have an orthorhombic unit cell of a≅b=0.532(4)nm, c=1.271(6)nm, by taking the a and b axes along the diagonal directions on the basal plane of the tetragonal archetype. Structural change occurred around 1018K. The weak reflections disappeared in the SAD pattern, suggesting that crystal is of the archetype above 1018K.