This paper proposes a mathematical model for multistage transmission expansion planning (TEP) considering fixed series compensation (FSC) allocation and N-1 security constraints for both transmission lines and FSCs. FSCs can increase the transmission lines transfer capacity, but the importance of using them in TEP is to dispatch the power more efficiently, resulting in a different topology with less investment cost compared with planning without FSCs. This problem is modeled as a linear mixed binary programming problem and is solved by a commercial branch-and-cut solver to obtain the optimum solution. FSC ismodeled as a fixed impedance located in the transmission lines in discrete steps and with an investment cost based on the prices of reinforcement lines. The results obtained using two known test systems show the economic benefits of using FSC in multistage TEP problems. © 2013 IEEE.