We present a systematic study of field-emission performance of prototype boron nitride (BN) nanocones using all-electron density-functional theory method. The effects of apical defects and doping/ adsorption on the field emission have been evaluated on the basis of magnitude of ionization potential (IP) and electron affinity ( EA). Among BN nanocones examined, two 120 degrees-BN nanocones, namely 120 degrees-4-B-N and 120 degrees-55-mol-B, have been identified as promising candidates for the field-emission electron source. Effects of the applied electric field on the electronic structures of BN nanocones have been investigated. In general, the electronic structures of BN nanocones can be significantly modified by a strong electric field, such as the reduction of the HOMO-LUMO gap and the change in density of states. The interaction between BN nanocones and applied electric field can be described by the second-order Stark effect. In addition, calculations show that the doping/ adsorption of an impurity atom results in higher IP or EA values, which is unfavorable to the field emission. Our study suggests that BN nanocones can be considered as alternative cold-emission electron sources.