We have developed a language called DCPP (Definite Clauses for Planning Processes) to represent knowledge concerning situations changing due to a process. The knowledge coded by DCPP is translated into definite clauses and forms a logic program that simulates the situation changes. Since data flows are bilateral in logic programming, the program works not only as a symbolic simulator for changing situations, but also as a planner for generating a process to achieve the required situation. As an example of knowledge encoding, we will show the start-up process for an electric power plant.