The effects of velocity boundary conditions on the structure of methane-air nonpremixed counterflow flames were investigated by two-dimensional numerical simulation. Two low global strain rates, 12 s−1 and 20 s−1, were considered for comparison with measurements. Buoyancy was confirmed to have strong effects on the flame structure at a low global strain rate. It was shown that the location where a top hat velocity profile was imposed is sensitive to the flame structure, and that the computed temperature along the centerline agrees well with the measurements when plug flow was imposed at the inner surface of the screen nearest the duct exit.