Abstract: Nanocrystalline material was chosen in the transformer design of the klystron modulator. The voltagecurrent vectors method was employed to measure the sample magnetic material, and the hysteresis curve, permeability and magnetic loss were obtained at frequencies of 20.03 kHz and 50 kHz. The magnetic capability of the large dimension toroid for application was measured by the pulse magnetization method. When the outer and inner dimensions of the toroid core were 220 and 80 mm, the hight was 60 mm, and the section was 4 200 mm2, the pulse width was 3.5~4.0 μs, the repelition rate was 50 Hz, the primary and second coils were 1 circle, the maximum voltsecond product was 0.007 V·s and the bias current was 8 A to maintain the toroid not saturated.