The (Zn 0.91 Mn 0.09 )O thin film annealed at 1000°C for 60min in N 2 atmosphere showed two kinds of Curie temperature (T C ) values of T C1 around 130K and T C2 above 300K while the as-grown (Zn 0.91 Mn 0.09 )O thin film showed typical ferromagnetism with the T C of 108K. It is expected that the increase of T C up to T C1 might be attributed to the enhancement of crystal magnetic anisotropy because the increases of (000l) peak for X-ray diffraction patterns and Mn 2+ -related emission for photoluminescence spectra were observed for annealed (Zn 0.91 Mn 0.09 )O thin films. The increase of T C above T C2 might be originated from the formation of nano-sized (Zn 0.91 Mn 0.09 )O islands because it was confirmed that the nano-sized (Zn 0.91 Mn 0.09 )O islands were formed after annealing treatment and that they revealed clearly the magnetic domains at 300K for the measurements of atomic force microscopy and magnetic force microscopy, respectively. The room temperature ferromagnetism in the nano-sized (Zn 0.91 Mn 0.09 )O islands is considered to be originated from the inhomogeneous distribution of Mn ions.