  The formation of charged pion condensate under parallel electromagnetic
fields is studied within the two-flavor Nambu--Jona-Lasinio model. The
technique of Schwinger proper time method is extended to explore the quantity
locating in the off-diagonal flavor space, i.e., charged pion. We obtain the
associated effective potential as a function of the strength of the
electromagnetic fields and find out that it contains a sextic term which
possibly induce weakly first order phase transition. Dependence of pion
condensation on model parameters is investigated.
