Palm-based polyurethane (PU) with soybean oil phosphate ester (PE) as a fire retardant was prepared. The effect of PE to the mechanical and burning properties of the PU was investigated. The PE was synthesized via ring-opening hydrolysis between o-phosphoric acid (o-H3PO4) and epoxidized soybean oil. The synthesis was conducted at 60-70 °C. The concentration of o-H3PO4 was varied at 2.5, 5.0 and 7.5 wt%. PE with 2.5 wt% H3PO4 showed the lowest acid value (2.8 mg·g-1) but high hydroxyl value (351.6 mg·g-1). PU containing PE (PU-PE) has higher density (60-61 kg·m-3) compared to control PU (49.7 kg·m-3). Burning rate decreased from 2.16×10-3 m·s-1 for control PU to 1.26×10-3 m·s-1, 1.06 10-3 m·s-1 and 0.6×10-3m·s-1 for PU-PE at 5, 10 and 15 wt% PE respectively. However, the addition of PE into the PU system gradually decreased the compression stress and modulus of the PU-PE.