Polyurethane based materials are usually preferred due to thermal and electrical insulation, lightness, durability, ease of design and production, high compression strength and corrosion resistance in many industries such as heat insulation and automotive industry. In this study, ammonium polyphosphate and pentaerythritol was used as flame retardant additives for polyurethane based composite materials. Boron derivative, ulexite, was also added at 1, 3, and 5 wt% as a synergist to improve fire retardancy performance of polyurethane composites. It is aimed to examine thermal, mechanical, physical properties especially flammability, by adding ammonium polyphosphate, pentaerythritol, and ulexite with different weight ratios into the polyurethane. It is shown that thermal conductivity, density, compression strength, modulus and Limited Oxygen Index values increased in polyurethane based composites. Flame retardants and ulexite addition have improved flammability properties of polyurethane composites.