Abstract: In this paper the dimensionless equations are derived from the method ofanalysis of directional unit system, and two examples are presented in illustration. The important priciple is suggested that the choice and treatment of the unit system in the similarity theory is quite differ from those of public unit systems such as SI system or engineering unit system, even though all they must conform to the Gauss' s rule of selection of unit system. For the former the flexibility and the particularity is necessary, because in the similarity theory the particular and actually phenomenon is investigated, and for the latter the invariance and the universality is necessary. as it is based on possibility of occurrance of all phenomena.