Seceond Harmonic Genration (SHG) and two photon luminescence are used to study the surface of metallic nanoparticles. We observed the first direct experimental evidence for the surface generated SHG signal from metallic nanoparticles. This observation clarified the confusion on the origin of the double frequency emission from metallic nanparticles in colloids. The SHG emission was then used to monitor the adsorption dynamics of thiol molecules on silver nanoparticle surface. The electronic defect on silver particle surface was also studied with two photon luminescence.