The aim of this study is to evaluate the physical, mechanical and flame retardant properties of particleboard made from oil palm trunk (OPT) with the addition of magnesium oxide as flame retardant. The physical and mechanical properties of the particleboard were evaluated based on Japanese Industrial Standard (JIS A 5908) and the flame retardant properties were determined according to International Standards Organization (ISO 4589). The particleboard was manufactured to a target density 0.80 g cm−3 with addition of magnesium oxide (0%, 10% and 20%). The results showed the thickness swelling and water absorption of treated particleboard decreased as the percentage of magnesium oxide increased. The untreated OPT particleboard without any flame retardant showed better mechanical strength. For thermogravimetric analysis (TGA), the treated OPT particleboard showed better thermal stability compared with the untreated OPT particleboard. The OPT particleboard treated with 20% magnesium oxide showed better flame retardant properties compared to the control and 10% magnesium oxide sample that satisfied the ISO 4589 standard requirement.