We calculate the one-loop free energy for two parallel D-branes connected by open bosonic (neutral or charged) string in a constant uniform electromagnetic (EM) field at nonzero temperature. For neutral string, external EM field contribution appears as multiplier (Born-Infeld type action) of one-loop quantities without the external EM field. The Hagedorn temperature is not changed if compare with the case of standard string gas in the constant electromagnetic field.