Creativity is an important topic in design research. Attempts have been made to develop methods and tools that can help designers become more creative. Yet how and why creativity occurs is still unknown to researchers. In this paper, we propose a theoretical model for creative design. This theoretical model builds on two postulates: 1) design reasoning follows a nonlinear dynamics, which may become chaotic; and 2) there is an inverse U shaped relationship between designer's mental stress and design creativity. In the first postulate, the nonlinear dynamics assumes the form of design governing equation and can be solved by Environment Based Design (EBD). The first postulate implies that design reasoning is sensitive to initial conditions, which are defined by the combination of design problem, design solutions, design knowledge, and other design related information. Since the major components in initial conditions may evolve simultaneously and are subject to continuous change during the design process, the design process is highly unpredictable. Some of the unpredictable solutions, which could be of high quality and useful, can be deemed creative. From this first postulate, three paths to creative design are derived, which specify how the initial conditions can be changed. The second postulate states that design creativity occurs when a designer is under a medium mental stress. Mental stresses are positively related to the workload associated with a design problem and negatively related to the designer's mental capacity. The workload is related to the complexity of the design problem and the amount of work in the design process whereas the mental capacity is related to the knowledge and skills required by the design process and to the designer's affect when dealing with the stresses arising from uncertainties and unpredictability of the design dynamics. To show how this theoretical model can be used to study design phenomena, an interpretation of the roles of sketching in design is presented.