Abstract: A theoretical model is established to investigate the influences of the group delay ripples (GDRs) of the fiber Bragg grating (FBG) on the characteristics of microwave-photonic filters using a coherent light source or an incoherent light source. Based on this theoretical model, the amplitude-frequency response and phase-frequency response of the microwave-photonic filter are investigated, while random and measured GDR distributions are employed. Results are then achieved for both coherent light source based and incoherent light source based microwave filters. For the coherent light source based filters, the GDRs result in a frequency shift to the notches of amplitude-frequency response. While for the incoherent light source based filters, the distortions to the profile of amplitude-frequency response and to the linear phase are observed due to the existence of GDRs. In addition, for the multi-tap microwave filters, the side-band suppression ratio and the extinction ratio are degraded under the influence of GDRs.