Exploring new topological materials with large topological nontrivial bandgaps and simple composition is attractive for both theoretical investigation and experimental realization. Recently, alpha tin (α-Sn) has been predicted to be such a candidate, and it can be tuned to be either a topological insulator or a Dirac semimetal by applying appropriate strain. However, freestanding α-Sn is only stable below 13.2 C. Herein, a series of high-quality α-Sn films with different thicknesses are successfully grown on InSb substrates by molecular beam epitaxy (MBE). Confirmed by both X-ray diffraction (XRD) and reciprocal space mapping (RSM), all the films remain fully strained up to 400 nm, proving the strain effect from the substrate. Remarkably, the single-crystalline α phase can persist up to 170 C for the 20 nm thick sample. The critical temperature where the α phase disappears decreases as the film thickness increases, showing that the thermal stabilization can be engineered by varying the α-Sn thickness. A plastic flow model considering work hardening is introduced to explain this dependence, assuming that the strain relaxation and the phase transition occur successively. This enhanced thermal stability is prerequisite for aforementioned roomtemperature characterization and practical application of this material system.The diamond-structured allotrope of tin (alpha tin [α-Sn]), also known as gray tin, has attracted increasing research interest recently for its topological characters when the cubic symmetry is broken. [1][2][3][4][5][6] The advantages of this material are attributed to its simple structure and large tunable nontrivial bandgap. [2][3][4][5][6][7][8][9][10][11][12]