The most stable conformer of 2-phenylpropanal is elucidated by supersonic jet spectroscopy. Only a single conformer of 2-phenylpropanal exists under jet conditions. This conformer shows two bands with red shifts of 32 and 65 cm(-1) when it is complexed with argon. The former corresponds to the mono-Ar complex and the latter to the di-Ar complex. From these results, it is concluded that the most stable conformer is 1, in which the CO group bisects the included angle defined by the C-CH3 and C-H bonds.