Persistent photoconductivity and optical quenching phenomena in boron-doped homoepitaxial diamond thin films grown on type Ib (100) substrates were investigated. Boron doping induced positive persistent photoconductivity (PPPC) and photocurrent gain upon the illumination of deep ultraviolet (DUV) light. A deep defect with a thermal energy of around 1.37 eV was deduced from the results of thermally stimulated current measurements. An optical quenching experiment revealed a threshold photon energy of around 2 eV. The difference between the thermal and optical energies of the deep defect suggests a large lattice relaxation. An absolute transient negative photocurrent (TNPC) was observed for zero or weak electric fields with DUV light illumination after optically quenching the PPPC. The absolute TNPC was due to transient electron filling of the charged nitrogen in the substrate. The combination of the nitrogen in the substrate and the boron-related defects in the epilayer was apparently responsible for the PPPC and TNPC effects. Therefore, the overall photoresponse properties of a thin epilayer deposited on a type Ib diamond substrate can be tailored by adjusting the concentration of nitrogen in the substrate and of boron in the epilayer.