Based on the equivalent medium theory and surface-plasmon-polariton (SPP) Bloch model, two kinds of novel metallic waveplates are analyzed: two-dimensional rectangular metallic grating and elliptical cylinder metallic grating. First, utilizing the finite-difference-time-domain (FDTD) method, we analyze the relation between characteristics of both waveplates in terms of the transmittance and phase retardation and parameters of the incident light in terms of the wavelength and polarization angle, especially transmission properties of both gratings functioning as quarter-wave plates. Simulation results show that both structures can act as quarter-wave plates when the input polarization angle is 75°. The corresponding transmittances of rectangular and elliptical cylinder gratings are 0.77 and 0.67, respectively. In addition, both gratings exhibit relatively flat transmission spectra within the wavelength range from 550 nm to 800 nm in the presence of different input polarization angles.