Biomass gasification was carried out in an External Circulating Countercurrent Moving Bed (ECCMB) gasification system. The system is composed of three reactors, a solid-solid co-current moving bed pyrolyzer, a gas-solid countercurrent moving bed gasifier and a riser-type combustor. A series of experiments were conducted with pine sawdust as feedstock and calcined olivine as solid heat carrier as well as catalyst. The operation conditions including the temperature of pyrolyzer, the steam/biomass (S/B) ratio, the particle size of feedstock, and the height of catalyst bed in the gasifier were investigated. The results show that the pyrolyzer temperature and S/B ratio are important factors for the product distribution. Gas yield and chemical efficiency are greatly increased with increased pyrolyzer temperature. The hydrogen content follows the same tendency. The introduction of steam improves gas quality and tar content of as low as 2 g/Nm(3) in the gas has been obtained. Both smaller particle size and higher catalyst bed height favor tar destruction, but impact no significant influence on the product gas composition. (C) 2012 Elsevier Ltd. All rights reserved.