Ultrafast electron diffraction (UED) has evolved to be a powerful tool for the study of structural dynamics with subpicosecond temporal resolution and subatomic spatial resolution. Recently, there have been intense research efforts toward femtosecond timing jitter and stability for improving the temporal resolution of UEDs, however, so far there has been no work showing long‐term (e.g., >1 h) stable timing for MeV‐level UED systems. In this article, a comprehensive timing synchronization method, based on optical‐radiofrequency synchronization and THz streaking, is demonstrated to maintain sub‐10‐fs long‐term timing stability for radiofrequency‐gun‐based MeV‐level UED, which results in 5.5 fs root‐mean‐square timing drift maintained over 4600 s. With high electron energy and low timing drift, the demonstrated capability is an important step toward studying ultrafast phenomena in samples with low scattering power, such as volatile gases and 2D materials. A THz‐streaking based timing system with sub‐10‐fs stability is implemented to operate MeV‐class ultrafast electron diffraction apparatus. Various timing methodologies, such as radiofrequency‐to‐laser synchronization, optical timing monitoring, electron dispersion control, are applied to enable THz‐streaking with few femtosecond resolution. With the THz‐driven streak camera, timing of ultrafast electron bunch is stabilized to 5.5 fs in rms.