Duplex-structured TC21 alloy samples were first solution-treated at a higher temperature in theα+βregion (940℃) with furnace cooling (FC), air cooling (AC), and water cooling (WC), followed by a second-stage solution treatment at a lower temperature in theα+βre-gion (900℃), and then finally aged at 590℃. The effects of the morphology and quantity ofαphases on the structure and properties of the TC21 alloy after the different heat treatments were analyzed. The in-situ tensile deformation process and crack propagation behavior were ob-served using scanning electron microscopy (SEM). The quantity of equiaxedαphases as well as the thickness of lamellarαphases reduced, the tensile strength increased firstly and then decreased, the elongation decreased with the increasing cooling rate after the first-stage solution treat-ment. The amount and size of lamellarαphases increased after the second-stage solution treatment because of sufficient diffusion of the alloy-ing elements, thereby leading to increased tensile strength. The amount of dispersedαphases increased after the third-stage aging treatment owing to the increase in the nucleation rate, resulting in a noteworthy strengthening effect. After the third-stage aging treatment, the first-stage FC sample exhibited better mechanical properties because it contained more equiaxedαandβtrans phases than the first-stage AC and WC samples.