We present second harmonic generations (SHG) of NLO polymers grown on Ag thin films at surface plasmon polariton (SPP) resonances. The Ag film itself exhibits surface nonlinear susceptibility and it is enhanced at the SP resonances. Our experimental results demonstrated that growing the NLO polymer layers on the Ag films was useful for further amplifying the conversion efficiencies of the SP-enhanced SHG. There was optimal polymer thickness for the SHG conversions, and approximately 40-fold maximum amplification was gained. The dependency of the SHG conversions on the polymer thickness was explained in terms of the bulk nonlinearities and the multiple reflections inside the polymer layer.