This study aims to clarify effects of isothermal bainitic transformation (IBT) temperature on microstructure and mechanical properties of a high Al–low Si δ-TRIP steel. For this purpose, the alloy was austenitised at the intercritical annealing temperature of 820°C for 600 s and then was isothermally cooled down into a salt bath and held at the temperatures between 350 and 450°C for 360 s. X-ray diffraction indicates that by increasing the IBT temperature, the volume fraction of retained austenite increases. In addition, it was found that the optimum combination of strength and ductility, i.e. tensile strength of 1046 MPa and total elongation of 54%, was achieved in the sample which was isothermally held at 450°C for 360 s.