Two- and three-body decays of charmed mesons are studied under the ``W-exchange'' dominance hypothesis. As for the two-body decays, the branching ratios for them are calculated in the scalar meson pole approximation. It is seen through a phenomenological analysis that the contribution of the annihilation diagram is much smaller than that of the W-exchange. This predicts that the F+to π+η decay is considerably suppressed although it is a Cabibbo favored decay. It is also seen that the new Cabibbo angle θc' which is defined in the charm changing currents is nearly equal to the old one θc.The soft meson technique combined with a linear approximation is applied to the three-body decays. The calculated value of B(D+to π+π+K-) reproduces considerably well the experimental value, but this method is not successful in the other three-body decays of D mesons. The branching ratios for these decays calculated by assuming that quasi tow-body decays contribute dominantly to these decays are almost consistent with the known data.