Abstract. To seek the standard form of a system of PDEs and to determine the dimension of its solution space according to the standard form are two important parts in the Riquier-Janet theory, where the dimension of the solution space is characterized by the number of parametric derivatives. Being so defined, the dimension of the solution space is related to the choice of order and is often incompatible with practical phenomena. In this talk, we try to introduce a new definition about the dimension of solution space, which can be uniquely determined due to independence of order and be compatible with physical phenomena.