Abstract: Based on the method of imprecision (MoI) and the robust design, uncertainty factors which influences the quality of products were considered during preliminary design. Using MoI mapping from a design variable space (DVS) to a performance variable space (PVS), an aggregation function was built and incorporated in different performance preferences on PVS to accomplish aggregation preferences for multi-attributes or more estimating persons. The example of gearbox was illustrated an application of the aggregation function in the optimal decision making of robust design. The results show that it is a convenient and practical method.