A systematic study of the influence of silver film thickness on the transmittance spectra through a silicon nitride photonic crystal slab (SiN-PCS) coated with silver films of various thicknesses is presented. The transmission spectrum through a bare SiN-PCS reveals guided resonances. When an ultrathin silver film (<10nm in thickness) is coated on a SiN-PCS, isolated silver nanoparticles are formed resulting in the suppression of guided resonances. In addition, a red shift is found in the optical transmittance spectra measured on SiN-PCS coated with a progressively thicker silver film. This may be attributed to the formation of localized surface plasmon resonances at the interface between silver nanoparticles and silicon nitride. A continuous texture is formed as the thickness of the silver film is increased up to 20 nm, resulting in a transition from localized surface plasmon resonances to surface plasmon polaritons. Furthermore, the surface plasmon polaritons that are formed externally (internally) are identified in SiN-PCS coated on one side (double sides) with a 20-nm-thick silver film.