The breakdown mechanism of a cylindrical electric heater is investigated by studying the uneven heating behavior of the heater through measuring the surface temperature variation of the heater when it is subjected to a boundary condition of constant heat transfer coefficients. The internal heating condition is inferred based on a surface temperature profile through heat conduction analysis. It is revealed that a commercial electric heater appears to generate uneven heat flux distribution on the surface, and the unevenness increases with heating-cooling cycles that the heater has experienced. The mechanism responsible for this behavior is discussed, which leads to a physical model accounting for heater breakdown.