Generative design system is a complex system that often focus on the technical and complex details thus away from building a system from the product designer’s thinking patterns. In addition, by focusing on the mechanism, the system is hard to construct and difficult to use for practical situation. Horizontal/vertical design thinking patterns are often used in product design practice, thus how designers make decision affect the generation reasoning. With the cognitive design researches on design thinking patterns, two research stages (experiment and implementation) are conducted and policy/generation rules are discovered during the experiment with 30 5-year design experts as focus group. An experimental system called thinking pattern-based generation system, a.l.a. ThinkGen is developed for testing the hypothesis.