Phase transition of nano-grain aluminium depicts many applications in automobile, aerospace, furniture, bio-medical, electronic devices, etc. due to its lightweight and non-corrosive nature. In order to improve the performance of aluminium in the above applications, we have investigated the evolutions of polycrystalline structures at various processing conditions such as up to 100 giga-pascal pressure and 1500 K temperatures by molecular dynamics simulations. We have found that the aluminium depicts liquid to bcc to fcc transition depending on temperature and pressure conditions. The size of nano-grains is highly dependent on the magnitude of applied pressures and temperature. The evolutions of various types of thermal-pressure engendered dislocations are observed during structural phase transitions. Solvent accessible surface area (SASA), volume of crystalline grain aluminium, adaptive common neighbour analysis and dislocation extraction algorithm have been used for quantitative characterization of polycrystalline aluminium. We believe that our results will provide new understanding to the researchers for the prediction of properties of metallic materials at extremely high pressure and temperature.