  Using the color evaporation model, the cross section for charmonium
production in p+p collision is calculated in quark-gluon plasma. The threshold
energy for open charms is given by the free energy potential from lattice
calculations, the initial charm quark pairs by the Pythia simulations, and
their time evolution by solving the Langevin equation. It is found that the
threshold energy which decreases with temperature reduces the cross section
while the invariant mass of charm pair which decreases by collisions enhances
it. As a result, charmonia production is suppressed by $30$\sim$50\%$ while
$J/\psi$ production is similar or enhanced compared to in vacuum.
