This paper is concerned with observer-based event-triggered control for cyber-physical systems (CPSs) subject to unknown periodic Denial-of-Service (DoS) jamming attacks, which aim to impede the communication on measurement and control channels. Firstly, in order to limit network resources, a resilient observer-based event-triggering communication scheme is designed under the unknown periodic DoS attacks whose DoS frequency and DoS duration are constrained. Secondly, stability criterion and stabilization conditions of the studied system are obtained by using piece- wise Lyapunov functional approach. Based on those conditions, the co-design method of the event-triggering parameters, the controller and observer gains is proposed. Finally, a practical example is employed to verify the effectiveness of the obtained theoretical results.