With the increasing integration of distributed energy resources (DER) and corresponding fluctuations, phase shifting transformers (PSTs) have been considered as important electrical components to increase transmission capacity by integrating PST with optimal power flow (OPF). However, due to vulnerabilities of supervisory control and data acquisition (SCADA) systems, applications in power systems may be subject to various cyber-attacks. In this paper, we discuss the feasibility of load redistri-bution (LR) attacks against OPF with PST, and quantitatively evaluate the effect of LR attacks on power systems. Specifically, the interaction between LR attacker and OPF with PST is formulated as a Stackelberg game, where the attacker and power system, acting as the leader and follower, attempt to maximize and minimize the operation cost of power system, respectively. By replacing the lower OPF optimization with its Karush-Kuhn-Tucker (KKT) condition, the Stackelberg game is simplified into a single-level mixed integer programming, which can be easily solved. Simulations in the PJM 5-bus, IEEE 14-bus and IEEE 30-bus test systems verify the feasibility of LR attacks and quantitatively evaluate the effect of LR attacks on the operation cost of power systems.