The prediction method of unscheduled direct maintenance cost (DMC) for civil aircraft structure was studied, and the present statuses of research at home and abroad were introduced. The damage sources of structure, the constituent elements and the influence factors of DMC were clarified. The prediction procedure and basic input were provided, and a prediction model of unscheduled DMC for civil aircraft structure was established based on the engineering design data. An example about a skin panel of forward-mid fuselage was given. A series of DMC values of the maintenance tasks from MSG-3, DTA and DSEA analysis were computed and the various DMC prediction results for metal and composite skin panel were compared and analyzed. It was shown that the application of composite materials can significantly reduce the unscheduled DMC value for this structure. The adjustment coefficient was introduced to optimize the prediction model according to aircraft design characteristics, operation environment and utilization rate, and the applicability and accuracy of the prediction model were improved.