The propagation of surface plasmon polaritons (SPPs) on Ag waveguides with two different widths is directly observed using dual-probe scanning near-field optical microscopy (DSNOM). We find that the waveguide structure strongly affects the propagation of locally excited SPPs. SPPs in a flat plane structure spread radially, whereas SPPs in a 3.4-mu m wide waveguide structure form two-dimensional interference fringes due to the multiple reflections at the side edges. The experimental results agree well with finite-difference time-domain calculations. The results suggest that the DSNOM technique can visualize the nanoscale characteristics of the SPP waves in various plasmonic waveguides. (C) 2012 Elsevier B.V. All rights reserved.