Abstract: As one of dielectric materials for embedded capacitors, Al powder/polymer composites have good application potentials due to low cost, easy processability and high dielectric constant. To lower the dielectric loss of these materials, the secondary passivation processes of Al flakes using air and tiny water respectively as the oxidants were studied. The structure and chemical composition of passivation films were characterized, and dielectric properties were evaluated for the passivated Al flakes and their polytetrafluoroethylene (PTFE) composites. The results show that several-nanometer thick secondary passivated film forms outside the self-passivated film of Al flakes, mainly consisting of Al₂O₃ and Al(OH)₃. The former contains main Al₂O₃, and the latter contains main Al(OH)₃. Air passivation occurs very slowly, the resulting secondary passivated films are thin but smooth, in contrast, tiny-water passivation proceeds quickly; the resulting ones are thick but loose. Tiny-water passivation exhibits better applicability in tailoring the thickness of passivated films than air passivation, so as to adjust the dielectric properties of Al flakes/PTFE composites.