Surface plasmon polaritons propagting along the silver nanowire was studied by near field scanning optical microscopy with twoprobes. In the experiment, one of the probe connected tolaser is positioned on one end of the silver nanowire to exicte surface plasmon polariton in near field, and the other probe scans along the silver nanowire to collect surface plasmon polariton in near field. The experimental result shows that surface plasmon polaritons are excited by the probe efficiently, and someof them reach the distalend facet of silver nanowire by propagating along the gap of silver nanowire and silica substrate. Finite element method is used to calculate the field distribution of surface plasmon polaritons modes on silver nanowire, which shows there are two surface plasmon polaritons modes with one as fundamental mode and the one as high mode. The fundamental mode of surface plasmon polariton, with its energy bounded to the gap of silver nanowire and silica substrate, suffers less scattering loss and can reach the other end of the silver nanowire. And the high modeof surface plasmon polariton attenuates quickly due to the scattering on the surface deffects. With the fundamental mode of surface plasmon polariton as information carrier, the efficiency of integrated optical device can be increased for its low propagation loss. What is more, the security of information transmission is improved since theenergy of the fundamental mode is bounded to the gap without scattering during propagtion, which is important in optical conmunication.