Shifts in the pull-in voltage of electrostatically actuated MEMS capacitive switches were characterized under pulse RF excitation, which allowed the electrical and thermal effects of the RF excitation to be separated. The resulted multi-physics model accurately predicted the pull-in voltage shift under different pulse powers and duty cycles. By comparing the power capacity of switches made of aluminum or molybdenum, a new figure of merit is proposed for selecting the optimum material for the fabrication of high-power MEMS capacitive switches.