The resonant interaction between the surface charge and the electromagnetic field of the light constitutes the surface plasmons polaritons (SPP) and gives rise to its unique properties [1]. Oscillations of such nature allow one to use plasmonic nanostructures as devices that control the optical radiation on micro- and nanoscale. Several techniques can be used to excite SPP. One of them is the use of a periodic corrugation of the metallic surface, which also leads to conversion of the polarization state of transmitted or reflected radiation by such structure [2]. Since the decay time of the SPP at optical wavelengths is about a few hundred femtoseconds therefore a change in the polarization state of light should occur at the same time scales. So it is of interest to study the ultrafast dynamics in the polarization state induced by SPP. If the laser radiation is a linear combination of p- and s-polarization than the dynamics of this state inside the pulse is not trivial, it becomes complicated because the SPP are excited only for p-polarized incident light. In this work the conversion of the polarization state induced by SPP into a single-shot femtosecond laser pulse reflected from a plasmonic crystal is studied. The samples were fabricated by thermal sputtering of metal layer onto a polymer diffraction grating with the period required for a given wavelength of used femtosecond laser. The reflection spectra of the sample demonstrated that in such structures SPP can be excited. The conversion of polarization state in the incident light is shown. During the ultrafast measurements a delay equal to 150 fs for p-polarized incident light compared with spolarized was found and it confirms that the state of polarization is modified on the scale of a few hundred femtoseconds in the plasmon-polariton nanostructures.