Periodic Event Triggered Control (PETC) is suited for applications where communication resources are scarce.As a new control strategy, the system theory in modeling and analysis of PETC is far from being mature.In this paper, Input-to-State Stability (ISS) theory is employed to analyze the linear state feedback periodic event triggered control systems.At first, the linear state feedback PETC system is considered as a discrete-time perturbed system, then the stability theorem of event triggered control system is deduced based on ISS theory of discrete-time perturbed systems, a Linear Matrix Inequality (LMI) form of the theorem is also given.At last, a method to determine event-triggering threshold is derived.Matlab simulation result verifies the correctness and effectiveness of our methods.