We document that monetary policy has asymmetric effects on investments by large and small firms in China. Large firms’ investment are highly responsive to monetary expansions, but less affected by monetary contractions. In contrast, small firms’ investments are less responsive to monetary expansions, but significantly affected by monetary contractions. We argue that this asymmetric responses of large and small firms stem from their differential access to credits in a two-tiered banking system. Large firms borrow from the big state-owned banks, which have a strong depositor base, whereas small firms borrow mainly from small banks which does not have a large depositor base and therefore rely heavily on the inter-bank market for financing their loans to small firms. We build a DSGE model with heterogeneous banks, heterogeneous firms, and an inter-bank market that is calibrated to the Chinese data. We show that the model’s quantitative predictions about the effects of monetary policy on large and small firms are consistent with the facts we documented.