With the development and increasing adoption of the Unified Modeling Language (UML), model-based testing has become one of the most researched and implemented testing areas. Among the different UML models, sequence diagrams are the most commonly adopted for representing the system interactions and communicating the functional requirements, and thus are often used to enable test case generation for a specific program. On the other hand, the “Design by Contract” technique consists of specifying the communication between the system components and the responsibilities and expectations of each one of them. This paper presents an approach to improve test case generation in model-based testing by encapsulating the different system constraints in one contract as an aspect and testing it independently as a unit. The testing of the different system base classes will be then performed through establishing a woven sequence diagram that is simpler to understand and which combines both base classes and aspects’ interactions, enabling an easier and more fruitful test generation process.