Abstract: Binary and ternary L10 type titanium alloys were rapidly quenched into thin foils by the hammer-anvil quenching technique. The nominal alloy compositions include Ti1-xAlx and (Ti1-xAlx)100-@#@ yMy, wherex = 0.5 to 0.6; y= 1 to 4; and M is V, Cr, Mn, Fe, Co, Ni, and Ge. Lattice parameters of Ll0 phases in these alloys were studied by X-ray diffractometer using the foils and their powders. Simultaneously, long-range order parameters of these Ll0 phases in the as-quenched foils and the annealed powders were determined from the superlattice intensity measured by X-ray diffractometer. Rapid quenching suppresses long-range ordering (S) during solidification and, thereby, reduces lattice distortion (c/a) in Ll0 titanium alloys. A small amount of alloying element has a large effect on thec/a ratio. The atomic size effect and probably substitutional site preference play an important role in such ac/a reduction. From the density measurement, it is found that antisite defects are a predominant mode in Al-rich off-stoichiometric TiAl compound alloys. Determined long-range order value is significantly low in as-quenched binary TiAl due to rapid quenching, and the highest ordering occurs at as-quenched Ti45Al55 alloy. The relationship betweenc/a andS in Ll0 Ti45Al55 may be written asc/a = 1 + 0.025 ·S.