The energy bandgap is an intrinsic character of semiconductors,which largely determines their properties.The ability to continuously and reversibly tune the bandgap of a single device during real time operation is of great importance not only to device physics but also to technological applications.Here we demonstrate a widely tunable bandgap of few-layer black phosphorus (BP) by the application of vertical electric field in dual-gated BP field-effect transistors.A total bandgap reduction of 124 meV is observed when the electrical displacement field is increased from 0.10 V/nm to 0.83 V/nm.Our results suggest appealing potential for few-layer BP as a tunable bandgap material in infrared optoelectronics,thermoelectric power generation and thermal imaging.