A new design methodology of embedded systems based on a user defined application specific processor, called a soft-core processor, and a new programming language, Valen-C, is proposed. In the initial design phase, a designer designs a system with a soft-core processor, RAM, ROM and logic circuits. Software on the system is written in Valen-C, in which the designer can specify the word length of each variable required for accurate computation in the program. After verifying the functionality of the initial design, we can optimize the system by changing several parameters of the soft-core processor. The total area of a system including the soft-core processor, ROM and RAM can be optimized by changing the word length. The proposed design method enables the designer to tune up the processor architecture for his application easily.