We present a novel method for converting channel plasmon polariton (CPP) waves in a waveguide into free-space optical waves. A grating decoupler in the CPP waveguide is designed by using grating equation. The CPP interacts with the grating decoupler and is re-radiated into the air. The effective index in the grating equation can be obtained by examining the dispersion characteristics of CPPs. In addition, a design strategy for efficient out-coupling is discussed. This works are believed to offer appropriate functionality for lab-on-a-chip sensing applications.