The ultra-wide bandgap semiconductor material α-Ga2O3 exhibits promising potential for applications in next-generation high-power electronics and deep-ultraviolet photodetectors. Its ultra-wide bandgap of 5.3 eV enables a high breakdown voltage and a potentially high Baliga’s figure of merit, while its rhombohedral corundum structure allows heteroepitaxy of α-Ga2O3 on inexpensive, isostructural sapphire substrates. The metastable nature of α-Ga2O3, however, results in a strong tendency for the films to revert back to the thermodynamically stable monoclinic β-Ga2O3 phase when grown beyond a critical thickness. Synthesizing thick, phase-pure α-Ga2O3 heteroepitaxial films with high crystallinity on sapphire substrates via metal-organic chemical vapor deposition (MOCVD) has proven to be a major challenge. This gives rise to the natural question: Can epitaxial growth of single-phase α-Ga2O3 with high crystalline quality be achieved via MOCVD, how does the critical thickness for phase transition of α-Ga2O3 depend on growth parameters, and what is the phase stabilization mechanism for α-Ga2O3 thin films grown by MOCVD? In this study, for the first time, we performed a systematic investigation on the stabilization of α-Ga2O3 on (10¯12) r-plane sapphire via MOCVD. An optimal growth window was identified at a reactor pressure of 15 Torr and substrate temperature of 575 ℃. The significance of O2 to alkyl molar ratio, as a growth parameter, and the introduction of an α-Ga2O3 nucleation layer deposited at a low growth rate were further elucidated in this work. These synthesis parameters were shown to play a substantial role in enhancing the phase purity, crystal quality, as well as increasing the critical thickness for phase transition of MOCVD-grown α-Ga2O3 thin films. Under the optimized growth conditions, we have demonstrated one of the lowest rocking curve full-width at half-maximum value ~ 1174 arcsec and the highest thickness ~ 1 μm ever reported on α-Ga2O3 heteroepitaxial thin films grown via MOCVD on r-plane sapphire substrates. Furthermore, we reveal that nucleated Ga2O3 with small grain size grown under high vapor supersaturation is key for stabilizing the α-phase in MOCVD. Based on the observation, we propose a strain-induced epitaxial stabilization mechanism for MOCVD-grown α-Ga2O3 films. Collectively, this study helped established an optimal MOCVD growth regime for α-Ga2O3 with high crystallinity comparable to conductive thin films grown by mist-chemical vapor deposition and pulsed laser deposition, paving the way for future work on controllable n-type doping of α-Ga2O3 films via MOCVD and the realization of α-Ga2O3-based electronic and optoelectronic devices.