Oil palm empty fruit bunch (EFB) contains the inorganic materials such as K, Na, Mg, and Ca, which cause some problems during fast pyrolysis reaction. In this paper, we report the effect of pretreatment by acid washing with dilute nitric acid to both remove the inorganic materials and increase the bio-oil production at fast pyrolysis in a bubbling fluidized-bed reactor. By pretreating dilute nitric acid at EFB, the total liquid yield enhancement was from 52.2 wt% to 63.9 wt% and the char yield reduction was from 39.9 wt% to 41.9 wt% compared to original EFB. Using various analytical tools including elemental analysis, thermogravimetric analysis, inductively coupled plasma atomic emission spectroscopy, pH meter, Karl-Fischer titration, gas chromatography, gas chromatography-mass spectrometry, and C-13 nuclear magnetic resonance, the properties and components of bio-oil in original EFB and acid-washed EFB samples were investigated. Through analytical tools, the biomass and bio-oil both in original and acid-washed EFB were analyzed to obtain various properties such as compositions, higher heating value, product yield, C-13 NMR spectra, and carbon number. (C) 2019 Elsevier Ltd. All rights reserved.