In the assembly modeling of complex products, it is not unusual facing an explosive growth of assembly relations, or called combination explosion. Through integrating object-oriented methods with the knowledge template based modeling, in this paper, a new methodology is proposed to deal with the complexity of the assemblies of large-scale products. As such, the templates with various assembly relations can be defined for complex products; constitutive parts or subassemblies in a product structure are defined as object classes; their features can be encapsulated, and defined or modified freely via the parameters of object classes to meet the requirements of a specific instance. The new method supports reusing knowledge and simplifying assembly modeling of complex products. In this paper, recent literatures on object-oriented methodologies and knowledge template methods for assembly modeling are reviewed, the rationales of object-oriented templates are discussed, and a method is proposed for creating assembly templates for complex products. In the case study included in this paper, the developed method has been applied to digital modeling of aero-engines, thus, validating its performance.