  We investigate the constraint on the split fermions in extra dimensions by
considering the universality of $W$ leptonic decays $W\to l_i \nu_i$, the
charged lepton decays $l_i \ra l_j\nu_i\bar{\nu}_j$, and the lepton flavor
violating process $l_i\ra \bar{l}_j l_k l_h$ where $l_i= e,\mu$ or $\tau$. For
the Standard Model (SM) background of $W\ra l_i \nu_i$, we extended the one
loop quantum correction to include effects of order $m_l^2/M_W^2$ and the Higgs
mass dependence. We find that in general the split fermion scenarios give rise
to a 4D effective Yukawa matrix of the Kaluza-Klein Higgs bosons is misaligned
with respect to the fermion mass matrix. This holds true also for gauge bosons
as well. This leads to decays of $l_i\ra \bar{l_j}l_k l_h$ at tree level and
muonium antimuonium conversion. Interestingly the leptonic universality of $W$
boson decays are not affected at this level.
