The structure characteristics and gasification activity of residual carbon in biomass ash from a 5 MW commercial updraft fixed-bed gasification power plant were investigated using a Raman spectroscopy, a pore structure analyzer and a thermo-gravimetric analyzer. In the investigated biomass gasification ashes, some fractions with a relatively high residual carbon contents could be probably recycled as fuel of gasifier. The crystalline structure of residual carbons in biomass gasification ashes were poorly organized, and the total active sites (sp(2) and mixed sp(2)-sp(3) bond forms) of residual carbons with were ordered as: fly ash > rice straw > bottom ash. The residual carbons in gasification ashes had a higher surface area and a higher gasification activity than those of rice straw. The residual carbons in bottom ashes had a lower surface area, more ordered carbon crystalline structures and less total active sites than those of fly ash. Consequently, the residual carbons in bottom ashes presented a lower gasification activity. The residual carbon which was generally considered as "unburnt carbon" in gasification ashes were probably originated from partly-gasified carbons and unreacted pyrolytic carbons. (C) 2016 Elsevier Ltd. All rights reserved.