Molecular dynamics simulations were performed to obtain the effects of pressure on the potentials of mean force between a pair of neopentane molecules immersed in water as well as in aqueous solutions containing trimethylamine-N-oxide (TMAO). It was found that the hydrophobic attraction acting between a neopentane pair in pure water increases with increasing pressure. However, the hydrophobic attraction diminishes by the addition of TMAO even at high pressure. This strongly suggests that TMAO does not counter the pressure induced protein denaturation by enhancing hydrophobic attraction among nonpolar groups.