Hydrogen-vacancy related defect (H1′) in chemical vapor deposition homoepitaxial diamond films has been investigated by electron paramagnetic resonance and cathodoluminescence. It is found that the concentration of H1′ significantly decreases as the dilution (CH4/H2) ratio decreases. It is also confirmed that the intensity of free-exciton emission (Iex) increases as the CH4/H2 ratio decreases. The complementary relationship between Iex and H1′ can be explained by considering that H1′ acts as a nonradiative recombination center which reduces the lifetime of free exciton and Iex. The suppression mechanism of H1′ is discussed by considering the balance between the growth rate and the annihilation rate of H1′ in the subsurface region.