<p>This paper proposes an online model-free Lyapunov Exponent (LE)-based Sliding Mode Control (LESMC) structure. The LESMC controls the system online based on its maximum LE without knowledge of the model structure or system uncertainties. The key to system control is to design the control law so that the LE becomes negative. The stability of the proposed control algorithm is proved in the paper and the control and design approaches are tested by simulation results on a nonlinear Single-Machine Infinite Bus (SMIB) system. Four scenarios are carried out in the simulation results section. The performance of the LESMC is analyzed, and the results are compared to the Model-Free Sliding Mode Control (MFSMC) and the Model-Free Intelligent Proportional-Integral Control (MFPIC) system structures as well. The results reveal the superiority of the proposed approach over the previous algorithms.</p>