Ln(2)MnNiO(6) were synthesized by the usual ceramic method for Ln = Sm, Eu, Gd, Tb and Dy and by the wet method for Ln = La, Nd Ho and Er. Heat capacities of these compounds were measured by using DSC and ACC. The thermal anomalies accompanying by magnetic transition from ferromagnetism to paramagnetism were observed for some rare earths and the magnetic contribution of heat capacity was estimated.