Double-perovskite oxides R2MnNiO6 (R=Pr, Sm, and Ho) were synthesized under mild hydrothermal conditions. Crystal growths of the samples were sensitive to the alkalinity, the reaction temperature and the time. The X-ray diffraction data show that all the compounds crystallize in the monoclinic structure of space group P2(1)/n. The purity and composition of the crystals are detected by ICP, SEM and EDX analysis. The magnetic studies for all the compounds indicated the dominance of ferromagnetic contribution and revealed a trivalent valence of both Mn and Ni with partial disorder. The T-C temperature of the R2NiMnO6 series is depended on the radius of R3+ atoms, which is consistent with the previous reported. (C) 2011 Elsevier B.V. All rights reserved.