Display omitted •High-performance PU foam nanocomposites were successfully fabricated.•A multi-methodological and modern approach was used to evaluate the flame resistance.•PU reinforced ZnO/MgO NPs and ATH exhibited an improvement inflame retardancy.•PU nanocomposites showed excellent thermal stability and compressive strength. In this work, we present the influence of zinc oxide (ZnO) and magnesium oxide (MgO) nanoparticles (NPs) combined with aluminum hydroxide (ATH) on the physicochemical properties and flame-retardant behavior of rigid polyurethane (PU) foam. Adding these kinds of NPs and ATH exhibit a significant influence on the morphological, structural, thermal stability, and mechanical properties of PU nanocomposites. Flammability including horizontal, vertical burning, and limiting oxygen index (LOI) tests were performed to evaluate the flame retardancy of PU nanocomposites. It could be observed that the effects of cell structureand density were close to the fire retardancy of PU nanocomposite foam. Whereby, a combination of ZnO and MgO NPs and ATH led to reduce cell size and increase density value of PU foams, thus playing the increased their flame retardancy. The flaming time of PU and its nanocomposites took 71 s and 59–62 s to self-quench the fire, respectively, and lost ∼84 % of their weight. The addition of 125 php ATH to PU nanocomposites led to a decrease in burning time to 9–10 s and a loss of ∼61 % of their weight. LOI value increased to 24 % with the incorporation of 5 php ZnO, MgO NPs, and 125 php ATH NPs into the PU foam.