GaN Cascode performance optimization for high efficient power applications is presented in this paper. Analytical equations of Cascode capacitance network (Ciss, Coss, Cgd) is demonstrated and the equations accuracy is verified through experimental measurement. Analysis shows that Cascode Cgd is determined by HV D-MISFETs Cds, LV Si FETs Cgd/Coss ratio, and extra zener diode capacitance. With low intrinsic capacitance HV D-MISFETs [1-2], proper LV Si FETs selection, and extra zener diode protection, optimization for Cascode switching figure-of-merit (FOM, Ron x Qgd) is well demonstrated. 8.8X lower switching figure-of-merit than commercial best-in-class Si SJ FETs [3] is achieved, double pulse test (DPT) and hard switching PFC system verification result all indicate that GaN Cascode is the promising solution and ready for next generation energy systems.