The three-dimensional effects of flame spread over thin solid materials were experimentally studied using a natural-convection-suppressing horizontal narrow-channel. In a sufficiently wide narrow-channel, the variation of flame spread against the width of the material sample showed different trends for different gas flow speeds and oxygen concentrations. The extent of three-dimensional effects was inversely proportional to the gas flow speed or its square. Near quenching extinction limits, the effects were significant because weak combustion is sensitive to a slight variation of heat loss and oxygen concentration. The effects may be due to different factors such as side heat loss, side oxygen diffusion, or both. Far away from quenching extinction limits, the effects were weak because vigorous combustion is insensitive to a small variation of oxygen concentration and heat loss. In all tests, the effects were limited to the samples of width less than 10 times of the diffusion length. Moreover, a higher oxygen concentration suppressed the effects at a lower gas flow speed. For sufficiently wide samples, in the most range of gas flow speeds, the channel width had almost no effect on flame spread. However, near extinction limits, the flame spread rate decreased with the increasing channel width. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.