The notational ambiguities in Feynman's calculus are all remedied here by setting a more natural foundation of the ordered exponential operators, which will be called briefly "expansional" operators in this paper. The essential point to be stressed is that the pure exponential opErator is merely a special case of the more wider class of operators, i.e., expansional operators, and the latter type of operators generally appears in quantum mechanics. A clear-cut distinction between these two types of operators is all-important. The so-called disentangling process is a device to decompose anyone expansional operator into a. product of some simple exponential operators. In the first place the general view-points concerning the expansional operators are presented, in which a rigorous organization of disentangling procedure is accomplished. Next are given some examples treating a ganerali.zed forced harmonic oscillator. where the transformation operator is completely disentangled and its representative Ylt'lds automatIcally the classical action in its compact form, which process constitutes the original intentIOn of this work. No accounts are given here concerning the quantized field theory.