A titanium dioxide modified expendable graphite (EG(T)) was prepared and characterized. The synergistic effect between EG(T) and ammonium polyphosphate (APP) on the combustion characteristics and thermal stability of rigid polyurethane foam (RPUF) were investigated through limiting oxygen index (LOI), vertical-combustion, microscale combustion calorimeter tests, and thermogravimetric/differential thermal gravimetric (TG/DTG) analysis. Results showed that EG(T) modified RPUF presented better thermal stability and flame retardancy than the normal expandable graphite (EG) modified RPUF. Addition of EG(T) improved the LOI value of 80RPUF/20EG(T) from 19.0% to 26.9%. Furthermore, the combination of EG(T) and APP caused the 80RPUF/10APP/10EG(T) to exhibit a LOI value of 27.8%, a vertical burning level of V-0 and a total heat release of 16.5 kJ g(-1). The research gave a body of evidence for synergistic performance between EG(T) and APP, and the effect of condensed phase mechanism was believed to be the main reason for such synergistic effect. POLYM. ENG. SCI., 58:2008-2018, 2018. (c) 2018 Society of Plastics Engineers