Water washing pre-treatment on empty fruit bunches (EFB) of oil palm was carried out for reducing some ash in the EFB to improve its quality prior to any thermal conversion process to yield better quality fuel. EFB was washed by varying some parameters such as the residence time and the amount of water used to study the effectiveness of washing with water. Total ash was reduced by about 24.9–70.3% in the EFB by various water washing pretreatments done in this work with an average ash content of 2.48 mf wt% achieved. The residence time would be expected to impact washing efficiency, which was described by the correlation between the percentage of ash reduction in the EFB and the incremental electrical conductivity (EC) of waste liquor. The use of tap water versus distilled water was also addressed for economic viability. Water washing pretreatment is an effective procedure in removing potassium and sodium in ash as they are proxies of ash in the EFB. Results showed that about 71% of potassium and 96% of sodium can be removed in ash. A thermogravimetric analysis (TGA) was performed under 100 ml min–1 nitrogen with a heating rate of 10ºC min–1 to investigate the thermal degradation characteristics of the unwashed and washed EFB.