The equivalent internal quantum efficiency (ηinteq) at 300 K of the near-band-edge excitonic photoluminescence (PL) peak in ZnO epilayers grown by plasma-assisted molecular beam epitaxy on Zn-polar ZnO substrates was directly correlated with the PL lifetime (τPL) for the first time. This relation seems to be universal for O-polar ZnO films grown by other methods. Present homoepitaxial ZnO epilayers grown above 800 °C exhibited atomically flat surfaces, and the best full-width-at-half-maximum value of (0002) ZnO x-ray diffraction ω-rocking curves was 17.6 arcsec. The high-temperature growth also led to a long τPL of 1.2 ns at 300 K. As a result, a record high ηinteq value (9.6%) was eventually obtained under an excitation density of 5 W/cm2 (He–Cd, 325.0 nm). The homoepitaxial Zn-polar ZnO films grown by molecular beam epitaxy are coming to be used for p-n junction devices.