An experimental study on the cellular instability of spherical propagation syngas flames with H2O addition was performed in a constant volume combustion bomb over a wide range of CO/H-2 ratios at elevated pressures. Schlieren images were recorded to observe and analyze the flame front evolution of spherical propagation flames. Results show that the cellular instability of syngas flames is promoted at atmospheric pressure and suppressed at elevated pressure with H2O addition for syngas mixtures with CO/H-2 ratio of 80/20. At elevated pressure, critical radius, beyond which flame front starts to accelerate, increases with H2O addition for syngas mixtures with higher CO/H-2 ratios of 65/35 and 50/50, while decreases for syngas mixtures with lower CO/H-2 ratios of 20/80 and 0/100. Critical Peclet number decreases with H2O addition for all of the mixtures. The acceleration exponent increases with the flame radius and reaches a constant value at some points. This implies that both self-acceleration and self-similarity regions exist in spherical syngas flames. The acceleration exponent in a self-similar region changes little with H2O addition due to the minor change of intrinsic flame instability.