Abstract: Basic parameters of the coplanar strip line on the substrate with finite dimensions are discussed in this paper. The coplanar strip line on the substrate with finite dimensions is a physical model, which is very closed to the engineering situation. The coplanar strip lines are analyzed using conformal mapping techniques. The exact solutions including the effective dielectric constant, the capacitance per unit length and the characteristic impedance are obtained. The results obtained in this paper have been compared with that of the coplanar strip line with infinite wide substrate. The relationship between the approximation error and the width of the substrate are given when using CPS3 as the approximation solution of CPS4. Multi-layered substrate coplanar strip line with finite dimensions is also discussed.