The thermal stability and the electrical reliability of HfO2 films with a blocking layer (BL) of Al2O3 on strained Si0.8Ge0.2 were studied. High-resolution transmission electron microscopy (HRTEM) indicates that BL keeps HfO2 amorphous after 700 °C annealing treatment. Energy dispersive X-ray spectroscopy (EDS) shows that BL can suppress Si diffusion in HfO2 films effectively. X-ray photoelectron spectroscopy (XPS) suggests that BL suppresses the growth of HfSiO and GeOx. Electrical measurements show that the reliability of the sample with BL is improved, including high capacitance density, low interface defect density, and small shift of flatband voltage after total-dose 60Co γ-ray irradiation.