  We discuss the semileptonic weak decays of $P\to P$ ($P$ denotes a
pseudoscalar meson). In these timelike processes, the problem of the nonvalence
contribution is solved systematically as well as the valence one. These
contributions are related to the light-front quark model (LFQM), and the
numerical results show the nonvalence contribution of the light-to-light
transition is larger than of the heavy-to-light one. In addition, the relevant
CKM matrix elements are calculated. They are consistent with the data of
Particle Data Group.
