To provide a reliable supply of electricity to consumers, power systems should be reinforced against various events. The traditional structure of power system vulnerability analysis involves a series of events that are often random and unintentional in nature. A power system are not only exposed to unintentional and random events; but also events occur deliberately and with the intention of jeopardizing the power system. Among the power system equipment, transmission lines due to the availability, are at risk of deliberate attacks more than any other power system equipment. Therefore, it is necessary to consider deliberate attacks in the power systems studies. Grid-scale electrical energy storage systems (E) are essential parts of the future power systems. The main characteristic of these systems is their ability for arbitrage of energy between different time intervals of the system operation. Distributed installation of E at a transmission system, in case of critical situations or outage of transmission lines, could be employed to make use of the E consumption/generation management as an alternative for transmission system and energy not served to maintain the power system stability. In this thesis, a preventive expansion planning is proposed to reduce vulnerability of transmission systems and intentional attacks damages. In this model coordinated expansion of transmission systems and E considering intentional attacks are studied. Two different models are used for modeling. In the first model, the problem implemented as a tri-level optimization problem. This is a defender-attacker-defender model. In this model, assuming given resources for an attacker, vulnerability of the proposed system is minimized. In the first level of this model, the power system planner is looking for an optimized planning scheme for the transmission network as well as the location and the capacity for the E. In the second level, the attacker by having complete knowledge of the network and system operator reactions and using limited resources, attacks to the transmission lines which attacking them will result in the maximum damages to the power system. In the third level, the system operator should make the best decision to minimize the load shedding at the time of operation. The third level optimization problem is a linear problem. Therefore, the second and the third levels of the optimization problem are modeled as an single level optimization problem. As a result, the initial tri-level optimization problem becomes a two-level nonlinear problem, that integration of this problem because of the nonlinearity of some constraints is not possible. Thus, a new approach to integrate and finding the optimal solution for the resulted two-level optimization problem is proposed. To demonstrate and clarify, the tri-level transmission network expansion planning considering a one load level is considered and analyzed. In the second model, due to the nonrandom and uncertain nature of deliberate attacks, by modeling these attacks as scenarios, a stochastic optimization model is used to solve the planning problem. In this model, although the vulnerability of the proposed system is not minimized, but using the proposed model, the network is robust against a set of attacks. The obtained models are implemented in Garver and modified IEEE 30-bus test systems. The numerical results are compared and validated. Numerical results show that the proposed models are able to reduce the vulnerability of transmission systems. Keywords: Coordinated expansion of transmission system and electrical energy storages, vulnerability of power system, two-level and tri-level optimization problems.