¡@¡@The countershaft transmission is a device connected between engine and driving wheel for transferring power. This device can provide variable gear ratios to maintain engine speed in certain operating range and is well applied for vehicle transmission systems for its high performance. In this thesis, the systematic design methodology for dual-clutch countershaft automatic transmission (DCT) is proposed. This methodology can provide a systematic design procedure for the gear box prototype, number of teeth, computer-aid program and preliminary layout. First, the fundamentals and gear-shifting operations for DCT are illustrated to establish the design requirements. Secondly, 12 kinds and 8 kinds for 7-speed DCT prototype are synthesized for the gear mechanism with FR type and FF type respectively. Then, according to both FR and FF types, calculate the proper number of teeth for ideal gear ratio and build the computer-aided calculating interface. Finally, the preliminary layouts of the above prototypes and number of teeth are built by CAD models. The result of this research will be beneficial for further newly DCT designs.