Two-dimensional (2D) hexagonal boron-carbon-nitrogen (h-BCN) atomic layers are expected to possess interesting properties complementary to those of graphene and h-BN, enabling a rich variety of electronic structures, properties and applications. Herein, we demonstrate a novel method to synthesize 2D h-BCN atomic layers with a full range of compositions by ion beam sputtering deposition under a mixed Ar/CH4 atmosphere. The h-BCN layers have been thoroughly characterized by various techniques, aiming at the determination of their structure evolution and properties. We find that homogeneous h-BCN layers consisting of graphene and h-BN nanodomains can be obtained at an appropriate C content, whereas too high or too low C contents result in the segregation of large-sized graphene or h-BN islands. Furthermore, the band gap of h-BCN layers slightly decreases with the increasing C content, while their electric properties can be tuned from insulating to highly conducting. This work provides a novel approach for synthesizing 2D h-BCN atomic layers and paves the way for the development of h-BCN-based devices.