By probing the resonances between a photonic band and an external laser field and their nonlinear changes in angle-resolved reflectivity, we show experimental evidence that the nonlinear optical changes in a two-dimensional photonic crystal waveguide with a Kerr nonlinearity are critically dependent on the dispersion nature and the group velocity of the photonic bands. The results agree well with the behavior predicted from band structures, indicating that the design of nonlinear optical properties of material systems is realistically possible by band dispersion and group velocity engineering.