In this paper, an extremum seeking control algorithm using sliding mode is proposed to find a setpoint which results in an extremum. A reference signal, a sliding mode, and a variable structure control input are designed so that the sliding mode is reached in a finite time, and then the reference signal and the cost function keeps increasing or decreasing for maximum and minimum, respectively in the sliding mode until the extremum is attained. The proposed extremum seeking algorithm ensures a satisfying control performance no matter how the system parameter changes.