“A waveform compensation circuit”, which generates compensation voltage to suppress ripple in current and flatten output current pulse of pulse forming network, is developed to increase the efficiency of pulse modulator with a Klystron. A waveform compensation circuit consists of compensation units connected in series. Each compensation unit consists of capacitances charged with compensation voltage, diodes, and FET switches. Compensation voltage is generated by turning on FET switches and turning eff diodes. Waveform shaping of output pulse of pulse forming network has been realized with compensation voltage up to 3kV. Flat top region of current pulse has been spread to double by increasing compensation voltage and improving fineness of timing control. Using the waveform compensation circuit, ripple in output current pulse of pulse forming network has been suppressed under ±0.1% of the peak current.