The hot electron buildup by second harmonic electron cyclotron resonance heating in the thermal barrier region of a tandem mirror has been studied by using a Fokker–Planck code with a self‐consistent potential profile taken into account. It has been found that the evolution of the hot electron population and the potential profile consists of two phases. In the first phase the rf diffusion is dominant and a quick increase in both the hot electron density and the mean energy is observed. No further increase in the mean energy is observed thereafter. The potential is deepest during the first phase. The second phase starts in the mean free time of the pitch angle scattering of hot electrons on cold electrons and ions. In this phase the hot electron population increases at the rate of the pitch angle scattering. The potential dip becomes shallow because of the accumulation of pitch‐angle‐scattered passing ions.