High strength, low density, and excellent corrosion resistance are the main properties making titanium attractive in a variety of applications. The crystal structures and structural transformations of titanium allotropes, which are of tremendous scientific and technological interest, have attracted a great deal of attention for many years. In addition to hexagonal close packed α-Ti, high temperature phase β-Ti with body-centred cubic structure and ω-Ti with the hexagonal structure, researchers have tried to find other metastable structures which can be maintained under extreme conditions through various methods. During the past decades, the ultrahigh pressure structures γ-Ti and δ-Ti are observed, and the room temperature ω-Ti and fcc-Ti can be obtained by high pressure torsion, laser shock, ultra-thin films, etc. The research progress of crystal structures and structural transformations of titanium were introduced. The structural stability and mechanism of structural transformations on titanium were elaborated.