The inner part (i.e., two-thirds of the diameter) of oil palm trunk (OPT) generally becomes waste because of its insufficient properties (e.g., low strength, low durability). Herein we attempted to use the inner part of OPT as a raw material of binderless particleboard in order to change this waste into an environmentally friendly value-added product. In this study, ammonium dihydrogen phosphate (ADP) was used as an additive, and the effect of ADP additions on the properties of particleboard manufacturing using the inner part of OPT was investigated. The ADP additions were varied from 0 to 40 wt%. First, thermal analyses were conducted to clarify the effect of ADP addition on the thermal properties of the OPT, and it was observed that the thermal properties of the OPT were significantly lowered with the increase in ADP additions. Based on these results, particleboards with a target density of 0.8 g/cm(3) were manufactured under the pressing condition of 180 degrees C for 10 min. The physical and mechanical properties of the particleboards were improved with the addition of ADP until 10 wt%. The water resistance was also significantly improved. It was suggested that part of the water-soluble components of OPT particles would change to water-insoluble components by the addition of ADP. Our findings demonstrate that the addition of ADP was effective for improving the physical and mechanical properties of particleboard using the inner part of OPT.