This paper presents the optimal maintenance of substation equipment. Along the equipment lifetime, the failure rate tends to increase gradually due to aging deterioration and daily operation. Therefore, the failure rate of equipment is proposed to be modeled by the appropriate Weibull distribution. Maintenance can keep equipment in good condition, but it incurs cost. The maintenance strategy typically consists of preventive and corrective maintenance. Both the maintenance costs are controversial, and consequently the maintenance strategy should be optimized for the minimum maintenance cost under accepted reliability condition. A Particle Swarm Optimization (PSO) algorithm is used to solve this objective problem. The obtained results demonstrate that the optimal maintenance can improve the reliability of equipment while the cost of the maintenance is satisfactory.