In order to detect turn-to-turn fault sensitively, a turn-to-turn fault protection based on the fitness value of particle swarm optimization (PSO) is proposed according to the influence of turn-to-turn fault on equivalent circuit of trans-formers. First, the fitness function for resistance parameter identification is constructed on the basis of the active loss equation of a transformer. Then, the parameter identification is achieved by PSO, and the minimum fitness value of search space is obtained. Finally, based on the minimum fitness value, a criterion is constructed to detect turn-to-turn fault. With transition resistance and measurement noise taken into account, the effectiveness of this protection is verified in simulations. And, the factors that possibly affect the sensitivity of this protection are analyzed. In addition, the sensitivity comparison of this proposed protection and the protection based on equivalent circuit equilibrium equation or excitation inductance is carried out. Some dynamic simulation experiments are accomplished at last. The conclusions can be drawn: this proposed protection can achieve high sensitivity in turn-to-turn fault detection and avoid the impact of parameter calculation error; the detection effect is not affected by load condition, and the detection sensitivity is still high under the influence of a certain amount of transition resistance; the detection effect is better than the other two protections mentioned above; dynamic simulation results verify the effectiveness of the protection.