Pyrolysis biochar is widely used as the catalyst for tar cracking,during biomass gasification. K and Ca in pyrolysis biochar affect the reforming of biomass tar under H2O or CO2 atmosphere significantly. In this paper, the transformation mechanism of H-form/K-loaded/Ca-loaded biochar structures and the reaction route of model tar compounds catalytic reforming were identified. The results show that during the reforming of model tar compounds in 15% H2O or pure CO2 at 800 degrees C, the release of K from biochar samples is nearly twice as that of Ca. More O-containing functional groups are formed on K-loaded biochar than Ca-loaded and H-form biochars. H2O or CO2 activation increases the lattice defects and surface functional groups in biochars to promote the combination between biochar and model tar compounds. The pathways for tar reformed in H2O or CO2 by K and Ca in biochar include direct homogeneous reforming and consumed by H2O or CO2 gasification on biochar surface. Tars containing heteroatoms are converted faster than those containing aliphatic chains and pure aromatic rings. On catalytic reforming of naphthalene and toluene with biochars, in 15% H2O atmosphere the effect of K is about 10% greater than that of Ca, while in pure CO2 atmosphere the effect of K is approximately 5% greater than that of Ca. (C) 2016 Elsevier B.V. All rights reserved.