Abstract: In wartime, in order to model the equipment maintenance and support process such that maintenance and support plan can be created, the support task should be decomposed to functions and activities. With restrictions in a support process, based on support activities, task decomposition granularity design model is developed in this paper by using the cohesion conception of modularized design in software engineering. By calculating the task and function cohesion metrics, and defining the analysis and evaluation criterion of the model, a new method is proposed for task decomposition granularity design. An example is used to verify the model and method, result shows that it is feasible and effective.