Nowadays, in spite of advanced communication technology, the post box collection activity is still operating by the agents of a country's postal service. Thus, this activity needs to be operated at minimum cost with smart decision making. The aim of this study is to find the optimal routing in post box collection activity with an objective to minimize the total traveling time. To deal with large-scale problem in the real-world practice, particle swarm optimization (PSO) algorithm are applied to solve the complexity of the problem. The experiments are executed using the case study of post box collection in Chiang Mai city, Thailand. The results showed that the proposed PSO can effectively find the new routing which helps to reduce the total travelling time by 11.14%, the total travelling distance by 7.11%, and the total cost by 13.86%.