Recently, a great deal of literature has been concerned with the security issues of wireless industrial control systems. However, there is still a lack of investigation on how the attacker should optimize its attack schedule in order to maximize the effect on the system performance due to the insufficiency of energy at the attacker side. This chapter fills in this gap from the aspect of control system performance. Especially, this chapter investigates the optimal denial-of-service (DoS) attack, which maximizes the linear-quadratic-Gaussian (LQG) control cost function under energy constraint. After analyzing the properties of the cost function under an arbitrary attack schedule, we derive the optimal jamming attack schedule and the corresponding cost function. System stability under this optimal attack schedule is also considered. Simulations are provided to demonstrate the effectiveness of the proposed optimal attack schedule.