Expansion of transmission networks can be beneficial for reducing operation costs, reducing trans-mission losses, improving voltage profile, and boosting power system security. On the other hand, when new transmission lines are connected to the transmission system, short-circuit level of buses in-creases, and consequently, the resulting short-circuit currents may exceed the circuit breakers’ ratings. If the standard short circuit level is violated, in the case of happening a fault, circuit breakers near the fault location may not be able to interrupt the fault current. In this case, to stop the fault current, the farther circuit breakers should operate, which puts a broader region of the power system out of service, and thus, the security of the entire power system is affected. Therefore, short circuit level should be kept within the standard range while expanding a transmission network. Considering short circuit cur-rent limitations can increase the cost of expanding a transmission network; On the other hand, changing the transmission network topology via transmission lines switching can be a suitable tool to reduce the short circuit level, with a negligible cost. In fact, by considering the possibility of transmission line switching, the increase in transmission network expansion costs due to short circuit level constraints can be reduced. In this thesis, dynamic and security-constrained transmission network expansion planning considering short circuit level constraints with the facility of transmission line switching is modeled in terms of a mixed-integer nonlinear optimization problem. The objective function of this model is to minimize the net present value of the total costs of constructing new transmission lines and power system operation costs along the planning horizon. Power system operation constraints under normal and single-contingency conditions (considering the N-1 security criterion) are modeled using DC power flow equations. To reduce the error in operation cost modeling, an approximation of transmission line losses is taken into account. Then, using linearization techniques, the nonlinear parts of the model are linearized, and the proposed model is recast as a mixed-integer linear programming problem. The proposed linear model falls into the category of NP-hard problems and may take a long time to solve. In particular, the dimension of the optimization problem becomes very large when single-contingencies through N-1 security criterion on all of the elements in the power system are taken into account. To reduce the proposed problem solving time, an iterative algorithm is used to gradually add contingencies to the problem. In this algorithm, a method to reduce the linearization error of short circuit level constraints is also presented. The proposed model is implemented in GAMS and simulated on a modified 6-bus Garver network as well as the IEEE 118-bus network. The obtained numerical results indicate the efficiency of the proposed model and the solution algorithm. Keywords : transmission expansion planning, short circuit level, transmission switching, N-1 security criterion, mixed-integer linear programming, linearizatio