•Superplastic behavior of near-alpha Titanium alloy (TA15) with equiaxed fine grain structure have been studied.•A wide range of thermal-mechanical tensile test performed at temperature range of 880–40 °C and strain rates range of 5 × 10–4–10–2 s−1.•A mechanism based unified viscoplastic constitutive model is established.•The proposed constitute model is implemented in FE software to check its validity against the superplastic forming experimental test for the multi-box die. Titanium alloy, TA15, has a high strength-to-weight ratio, high weldability, and superior creep resistance at high temperatures up to 550 °C. TA15 is difficult to deform, especially for forming complex-shaped large-scale web–rib components, due to its low plasticity, large inhomogeneous deformation and narrow processing window. The objective of this research is to model the superplastic mechanisms in TA15 alloy with equiaxed, fine grain structure, and applying the proposed constitutive model to investigate the maximum grid aspect ratio, that can be achieved in superplastic forming (SPF), for a TA15 sheet with an initial thickness of 1.2 mm. Thermo-mechanical tensile tests are conducted first to characterize the superplastic behavior of the material in the temperature range of 880– 940 °C and the strain-rate of 0.0005–0.01 s−1. A set of mechanism-based unified visco-plastic constitutive equations has been proposed and calibrated based on the results of stress-strain data. A gradient-based optimization method is applied for the calibration of constitutive equations. The constitutive model is incorporated into FEA code through creep subroutine to check the validity of the proposed material model against the experimental SPF test of a multi-box die. Predicted sheet thickness and thinning in a die entry radius region at the end of forming are examined in detail. Preliminary results show a good agreement between the computational and experimental results.