Capacitor loss in pulsed power systems has become an important issue for thermal management, especially when the operating rep-rate and energy per pulse are getting higher and higher. It is practical to analyze the loss of a capacitor using a capacitor series equivalent circuit model in this pulsed power application. The capacitor loss is directly related to the equivalent series resistance (ESR) of the capacitor. In the paper, the formula to estimate the capacitor loss is presented. It is proportional to the energy per pulse, rep-rate, and capacitor dissipation factor (tandelta) at the certain energy transfer frequency of the circuit. The charging and discharging frequencies of the capacitor might be different. Both of them will produce losses, and contribute to the total capacitor loss. The analysis of a capacitor bank with capacitors in parallel or in series is presented as well. Also in this work, the parameters of different types of capacitors were measured, including film capacitors and high voltage ceramic capacitors, using a Agilent 4285 A Precision LCR Meter up to 13 MHz. The influence of temperature on capacitance and dissipation factor was investigated by sinking the capacitor in oil under controlled temperatures. The results are very helpful to estimate the losses of capacitors in Cymer's laser pulsed power systems, and to provide design guidelines to the next generation of 6 kHz laser power systems.