We present a charge transport study on single naphthalene and anthracene molecular junctions wired into Ag and Cu electrodes using mechanically controllable break junction technique at 100 K under ultra-high vacuum condition. In particular we focus on effect of metal-pi interaction on the formation probability of the molecular junctions. We found that the single molecular junctions of the acene molecules (e.g. naphthalene and anthracene) exhibit highly conductive character below 0.2 G(0) (G(0) = 2e(2)/h). The acene molecular junctions displayed formation probability of ca. 20% for Ag system and >40% for Cu system. The high formation probability of the molecular junctions with respect to benzene/Au junctions can be qualitatively explained by size effect, in which larger molecules of the naphthalene and anthracene can effectively bridge the gap between metal electrodes compared with small molecule such as benzene. The acene/Cu junctions displayed higher formation probability than the acene/Ag junctions. This result demonstrated that not only the size effect but the degree of the metal-pi interaction have to be taken into account to quantitatively evaluate the formation probability of the molecular junctions for Ag and Cu system. (C) 2015 Elsevier B.V. All rights reserved.