Nano-imprint lithography possesses the advantages of high throughput, sub-10-nm feature and low cost. In NIL, the mold filling is subjected to the applied imprinting pressure, temperature and time. Incomplete mold filling causes a detrimental effect on the final imprinted pattern dimensions. The monitoring system of imprinting is essential to control the imprinting parameters properly. Up to now, no high-sensitivity monitoring of filling rate and end point has ever been proposed. In this study, the authors apply the surface plasmon resonance to monitor the filling rate and end point during imprint process. The mold contains a layer of glass of high refractive index, a metal thin film and the pattern of low refractive index. In addition, the imprinted polymer is selected considering its refractive index, which should be lower than the glass layer of mold. When the filling rate varies, it will affect the SPR behavior, including the measurable reflectivity change and resonance angle shift. The analysis results reveal that the resonance angle is truly proportional to the filling rate. When the filling rate varies from 50% to 100%, the SPR angle shifts more than 5 degree. The analysis demonstrates this innovative method for monitoring of filling rate is effective with high sensitivity.