Abstract: In this work, expandable graphite (EG) and ammonium polyphosphate (APP) were used as flame retardants, and transition metal oxides (Cu₂O, Fe₂O₃, Ni₂O₃, Co₂O₃) were used as synergists. The flame retardants and synergists were applied to prepare for flame-retardant rigid polyurethane foams (RPUF) at the same total contents of 30 php and the same mass ratio among APP, EG and transition metal oxides (15: 13: 2). Then, tests of limiting oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter were used to investigate the flame retardancy and smoke suppression properties of RPUF. The experiment results of LOI and UL-94 showed that the flame retardancy of RPUF/APP/EG was improved more or less by addition of transition metal oxides, in which only the addition of Cu₂O could improve the LOI value to 25.5%, exhibiting better synergistic effect with EG and APP than Fe₂O₃, Ni₂O₃ and Co₂O₃. Besides, the addition of Cu₂O could further decrease the total heat release and total smoke production of RPUF/APP/EG by 22% and 20%, respectively.