The atomic-scale microstructure and electron emission properties of boron and sulfur (denoted as B-S) codoped diamond films grown on high-temperature and high-pressure (HTHP) diamond and Si substrates were investigated using atom force microscopy (AFM), scanning tunneling microscopy (STM) and current imaging tunneling spectroscopy (CITS) measurement techniques. The films grown on Si consisted of large grains with secondary nucleation, whereas those on HTHP diamond are composed of well-developed polycrystalline facets with an average size of 10–50 nm. Large tunneling currents were observed at some grain boundaries, and the emission character is better at the grain boundaries than at the center of the crystal. The codoped films grown on HTHP diamond have an almost uniform electron emission efficiency at grain boundaries or crystalline facets, which indicates that the doped atoms are uniformly distributed in the films. The local I-V characteristics for films deposited on Si or HTHP diamond substrates indicate n-type conduction.