  By using slave particle (slave boson and slave fermion) technique on the
Bose-Hubbard model, we study the finite temperature properties of ultracold
Bose gases in optical lattices. The phase diagrams at finite temperature are
depicted by including different types of slave particles and the effect of the
finite types of slave particles is estimated. The superfluid density is
evaluated using the Landau second order phase transition theory. The atom
density, excitation spectrum and dispersion curve are also computed at various
temperatures, and how the Mott-insulator evolves as the temperature increases
is demonstrated. For most quantities to be calculated, we find that there are
no qualitatively differences in using the slave boson or the slave fermion
approaches. However, when studying the stability of the mean field state, we
find that in contrast to the slave fermion approach, the slave boson mean field
state is not stable. Although the slave boson mean field theory gives a
qualitatively correct phase boundary, it corresponds to a local maximum of
Landau free energy and can not describe the second order phase transition
because the coefficient $a_4$ of the fourth order term is always negative in
the free energy expansion.
