The pyrolysis bio-oil from rice husk was upgraded in sub-and super-critical ethanol using HZSM-5 as catalyst. The results showed that super-critical upgrading process performed more effectively than sub-critical upgrading process. Acidic HZSM-5 facilitates esterification in super-critical ethanol to convert acids contained in crude bio-oil into various kinds of esters. Stronger acidic HZSM-5 (low Si/Al ratio) can facilitate cracking of heavy components of crude bio-oil more effectively in super-critical upgrading process. The residue of distillated upgraded bio-oil from super-critical upgrading process decreased evidently, compared with that of distillated crude bio-oil. This work proved that crude bio-oil can be effectively upgraded in super-critical upgrading process with the aid of acidic catalyst. (C) 2009 Elsevier Ltd. All rights reserved.