Exciplex formation in the van der Waals mixed dimer of naphthalene and 2-methoxynaphthalene has been investigated by using laser-induced fluorescence spectroscopy in supersonic jet. In contrast to the barrierless excimer formation in the S-1 state of naphthalene homodimer, the exciplex formation in the mixed dimer occurs with a vibrational energy barrier of 466 cm(-1). The energy gap between the S-1 states of two molecules in the mixed dimer is 1000 cm(-1). Therefore, the contribution of the exciton resonance interaction to the stability of the exciplex state is considered to be insignificant. Charge transfer interaction and the mixing of the charge transfer state with the locally excited state due to orbital overlap between two molecules at suitable geometry are proposed to be the decisive factors in formation of the exciplex.