This paper proposes the high-frequency mathematical model for the common-mode noise voltage of a DC–DC buck converter. The high-frequency equivalent circuit considering the parasitic impedance is proposed, and then the impedance path of the common-mode noise is analyzed using this high-frequency equivalent circuit. The common-mode noise source generated by the switching device is divided into trapezoidal and ringing voltage noise sources and mathematically modeled. For high accuracy at high-frequency, a mathematical model of common-mode noise sources including the difference between rise and fall time is proposed using Fourier transform. The common-mode noise voltage is modeled by multiplying the impedance path and the noise source. In the results of the simulation, the mathematical model and experiment, the resonance frequency and the magnitude matched with less than 5% error. Through this, the common-mode noise is predicted accurately using the high-frequency mathematical model proposed in the paper.