Quantitative evaluation of rock drillability is of vital importance for selecting drilling bits and optimizing drilling parameters. The existing drillability evaluation methods are mostly designed for balanced and near-balanced drilling situation (roughly 0 bottomhole differential pressure), while investigations of under-balanced drilling are rare. In this work, rock drillability and acoustic velocity testing devices are used to investigate rock drillability in an under-balanced drilling situation. The experimental results showed that bottomhole differential pressure produces a great impact on both rock drillability and acoustic characteristics. Mathematical statistics methods have been applied to analyze the correlations between rock drillability and acoustic characteristics as well as between differential pressure and acoustic characteristics, and to build a model for forecasting rock drillability under various bottomhole differential pressures. The model provides a sound approach to quantitative assessment of rock drillability based on log data.