Geometrical effects on the flame spread over thermally thick combustibles in a narrow channel were experimentally investigated. Two series of experiments were conducted with two kinds of fuel geometry to form a narrow channel: parallel plates and cylinder. Both channels were made of polymethyl methacrylate, which also served as a combustible specimen, and the channel height varied from 1 mm to 10 mm. An oxidizer flow was supplied at specified velocities of 30 cm/s or 90 cm/s. The flame spread through the channels in the direction opposite to the oxidizer flow was observed. Although the spread rates had similar trends, there was a slight difference between their values at the same conditions. The effective velocity and the excess-volume ratio were evaluated to explain the geometrical effects. Accordingly, it is suggested that the variation of the velocity profile due to the channel geometry is the main factor affecting the spread rate.