The epicyclic gear mechanisms are widely used in the automatic transmissions for automobiles, and can provide several speed ratios by the controls of the input and fixed links with clutches and brakes. In order to increase the number of speed ratios, two or three embedded clutches are adapted in the automatic transmissions. However, the design of automatic transmissions with one embedded clutch has been hitherto ignored. Thus, the purpose of this work is to develop a systematic methodology for the design of automobiles automatic transmissions with one embedded clutch. First, the design specifications are established based on the requirements of the epicyclic automatic transmissions. Next, a procedure of the conceptual synthesis is developed to enumerate the epicyclic gear mechanisms with one embedded clutch. The corresponding atlas of the epicyclic gear mechanisms with two fundamental geared entities are established. Finally, a method is created to arrange the clutches and brakes into epicyclic gear mechanisms. Then the numbers of the teeth of gears are calculated by an analytical way. The result of this research shows that five five-speed automatic transmission and one seven-speed automatic transmission are synthesized. The associated efficiency for each speed is greater than 95%. This demonstrates the feasibility of the proposed methodology.