Abstract: During the optimization of transmission network planning which is regarded as a problem of multi-objective mathematical optimization, the three influencing factors are taken into account, i.e., investment cost, reliability and its effect on environment. To overcome the drawbacks of conventional mathematical optimization, such as local optimization and dimension disasters, the particle swarm optimization (PSO) is firstly introduced into the optimization of transmission network planning, and an optimal scheme is thus formulated. With a practical project exemplified, the PSO shows its methodological feasibility and efficiency. Compared with the existing planning methods, PSO provides the advantage of shorter time in searching and results in a solution closer to the optimum one.