The group velocity matching and group velocity dispersion have been investigated for noncollinear optical parametric amplification with periodically poled KTiOPO4 (KTP). A general mathematic model for evaluating the spectral bandwidth of optical parametric amplification (OPA) is developed. The spectral bandwidth is determined by expanding the wave vector mismatch in a Taylor series and retaining terms through second order. If the first-order term vanishes, noncollinear interaction tree waves are group matching. And the spectral bandwidth is determined by group velocity dispersion. The dependences of gain bandwidth on crystal length and pump intensity are also studied. Selecting appropriate grating period, wider tunable range can be obtained with group velocity matching. The parametric bandwidth is relating to crystal length, noncolinear angle, group velocity and group velocity dispersion. Because group velocity is matched, the parametric bandwidth is very large. The effective nonlinear coefficient is nearly equal to d33. And maximum effective length and conversion efficiency are enhanced largely.