A new rhombohedral phase (termed R′) in a solution-aging-treated titanium alloy (Ti-4.5Al-6.5Mo-2Cr- 2Nb-1V-1Sn-1Zr, wt.%) was identified. Its accurate Bravais lattice parameters were determined by a novel unit cell reconstruction method based on conventional selected-area electron diffraction (SAED) technique. The orientation relationship between R？ phase and BCC phase was revealed. The results show that the R′ phase is found to have 48 crystallographically equivalent variants, resulting in rather complicated SAED patterns with high-order reflections. A series of in-situ SAED patterns were taken along both low- and high-index zone axes, and all weak and strong reflections arising from the 48 variants were properly explained and directly assigned with self-consistent Miller indices, confirming the presence of the rhombohedral phase. Additionally, some criteria were also proposed for evaluating the indexed results, which together with the Bravais lattice reconstruction method shed light on the microstructure characterization of even unknown phases in other alloys.