Rigid polyurethane foam (RPUF) composites filled with various expandable graphite (EG) or/and whisker silicon (WSi) loadings were prepared with the same density (0.08 g/cm(3)). When the WSi content reached 10 wt %, the composite had the best mechanical properties, the values of compressive strength and modulus reached 0.74 and 21.0 MPa, respectively. Compared with only EG filled RPUF, the addition of 10 wt % WSi to EG/RPUF would also bring out improvement on their flame retardant and mechanical properties. Maintaining EG 20 wt %,, the compressive strength increased from 0.17 (EG/RPUF) to 0.39 MPa (WSi/EG/RPUF), the compressive modulus increased from 9.32 (EG/RPUF) to 10.1.1 MPa (WSi/EG/RPUF), and the limiting oxygen index value increased from 28 vol % (EG/RPUF) to 32.5 vol % (WSi/EG/RPUF). The thermogravimetric analysis showed that the addition of EG and WSi particles improved the thermal stabitity of the composites. The dynamical mechanical analysis (DMA) showed that EG and WSi particles led to up-shift of the glass transition temperature. Moreover, 10 wt 9% EG and 10 wt % WSi filled RPUF had the highest storage modulus, as the EG and WSi exhibited a desirable synergetic effect on flame retardant properties and dynamic mechanical properties of RPUF composites. The morphologies of the composites were analyzed with scanning electron microscopy. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 3871-3879, 2008