With the increasing system clock, a clock circuit will cause an amount of electromagnetic interference. To reduce EMI of the products, various EMI strategies have been developed. In the thesis, we study the suppression of electromagnetic interference of a clock circuit by using the spread spectrum technique. The basic idea of the spread spectrum is to slightly modulate the frequency of the clock signal and the energy of the signal will be dispersed to a controllable range to reduce the peak energy of each harmonic wave in the spectrum, and the products can pass the electromagnetic compatibility test more easily. We obtain the attenuation factor of spread spectrum from the theoretical derivation, including modulation index and modulation profiles. From the numerical simulation, we verify that spread spectrum technique can suppress the peak energy. We propose the attenuation formulas which can control the attenuation of every frequency point. To verify our findings, we use a spread spectrum clock generator from market to perform measurement. The trapezoidal waveform can be used to represent a clock circuit. Its waveform includes rise time and duty cycle. We will discuss the influence of rise time and duty cycle on the spread spectrum technique. Shorter rise time will cause high order harmonics in the high speed clock circuit. We verify that spread spectrum technique can suppress high order harmonics from both the simulation and experiment. Because every harmonic can be suppressed, the spread spectrum technique has the good suppression effect for the whole system.