  We calculate the masses and leptonic decay constants of the heavy vector
quarkonia, $J/\psi$ and $\Upsilon$ mesons at finite temperature. In particular,
considering the thermal spectral density as well as additional operators coming
up at finite temperature, the thermal QCD sum rules are acquired. Our numerical
calculations demonstrate that the masses and decay constants are insensitive to
the variation of temperature up to $T\cong 100 ~MeV$, however after this point,
they start to fall altering the temperature. At deconfinement temperature, the
decay constants attain roughly to 45% of their vacuum values, while the masses
are diminished about 12%, and 2.5% for $J/\psi$ and $\Upsilon$ states,
respectively. The obtained results at zero temperature are in good consistency
with the existing experimental data as well as predictions of the other
nonperturbative models. Considerable decreasing in the values of the decay
constants can be considered as a sign of the quark gluon plasma phase
transition.
