In t h i s p a p e r, t h r e e -d i m e n s i o n a l full wave analysis of tapered microstrip lines is carried out in the time domain. The method used is the conformal grids FD-TD method. Computed results are compared with experimental ones and those obtained by the finite element method. The results agree well. As a result, it is shown that the propagation c h a r a c t e r i s t i c s a r e v e r y d i f f e r e n t from t h o s e obtained through two-dimensional modeling.