The interactions between the optical light and sub-wavelength structures in metal film have sparked great interest on surface plasmon polaritons (SPPs). One effective way to interpret these phenomena is to make use of the phase shift between the SPP excited and the incident wave. Many theoretical and experimental studies have shown that this shift approximate to π is intrinsic at normal incidence. In this paper, the phase shift at oblique incidence is analyzed and used to interpret the transmission effect for the first time to our knowledge. A simplified 2D scattering formulation using vector finite element method is set up. Simulations show the phase shift can be approximate to π over a large range of incidence angles. With this shift, the conditions for the enhanced or suppressed transmission and the corresponding destructive or constructive interference of SPPs at oblique incidence are achieved. The transmission effect for incidence angle not satisfying these conditions, as well as its potential applications is also discussed.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.