Polyethylene (LLDPE) compounds containing 10 wt-% graphite fillers were rotationally molded into flat sheets. Flame retardancy was studied using cone calorimeter tests conducted at a radiative heat flux of 35 kW m-2. Only the expandable graphite, an established flame retardant for polyethylene, significantly reduced the peak heat release rate. Compared to the neat polyethylene, it was easier to ignite the LLDPE composites containing carbon black, expandable graphite and exfoliated graphite. However, rather unexpectedly, the inclusion of flake graphite increased the time to ignition by up to 80%. Simulations conducted with the ThermaKin numerical pyrolysis software suggest that increased reflectivity was mainly responsible for this effect. The behaviour of the compounds in micro-flammability tests did not reflect these results. The reasons for the discrepancy will b.e discussed