Maintenance function is one of the most important activities to keep machines and equipment of a plant in satisfying operating conditions for maintaining their reliability at the desirable levels. The six-sigma based methodology proposed in this paper for maintenance function is to meet two specific objectives – to measure and evaluate its capability over time, and to improve its performance through target setting and benchmarking critical maintenance related controllable variables and parameters. Six-sigma based maintenance methodology has huge potential for reducing costs and risks. It is expected to achieve error-free responsive maintenance system in a technology-based complex and dynamic environment.A typical performance based model for maintenance essentially depends on measuring and controlling variables. The maintenance performance indicators with respect to various maintenance groups and sections need to be identified for measuring and evaluating maintenance capability. In this paper, six-sigma methodology within the DMAIC framework is proposed to identify the maintenance activities and their capability. A continuous improvement in maintenance performance is main objective of this research. The performance measures, capability indices, key process maps, and improvement strategies are outlined in this paper.