This paper investigates new determinants of US bank capital structure over 2000-2013 using instrumental variable regression, fixed-effects models and quantile regression. We find that higher competition and securitization separately reduce bank capital ratios. Additionally, we exploit possible channels underlying these results. More importantly, we show that competition, when interacted with securitization, has a significant implication for the ongoing debate on bank capital. Interestingly, deeper investigations emphasize a positive interaction effect on less-capitalized banks compared to highly-capitalized banks. Overall, our findings introduce previously undiscovered empirical evidence to the general theoretical models about capital structure, and implement some important policy implications.