A composite film containing Al2O3 nanoparticles was successfully anodized on the AZ31 alloy using an alkaline NaOH-Na2SiO3 solution. The formation process and the mechanism of the nanoparticles were investigated by means of XRD, SEM and EDS characterization. The addition of nanoparticles does not change the basic process of anodizing magnesium alloys. The thickness change of the oxide films is consistent with the voltage variation and the oxidation time. During the first 5 seconds of anodization, the sample weight decreases, indicating that Mg dissolution dominates this process. Subsequently, the sample weight begins to increase and the film development plays an important role. The oxide film is formed by Al2O3 nanoparticles absorbed on the surface of the magnesium matrix. The nanoparticles have been involved in the growth process of the oxide films. The composite oxide film is composed of MgO, Mg2SiO4 and a-Al2O3 phases. In the process of composite oxidation, Al2O3 nanoparticles can fill in the pores and the cracks of the oxide films after being absorbed on the surface of the film. In addition, they can also be continuously captured in the oxide film, forming an oxide film structure reinforced by Al2O3 nanoparticles and leading to enhanced performance.Avtorji opisujejo uspe{ni nastanek kompozitnega filma, ki vsebuje Al2O3 nanodelce na povr{ini Mg zlitine AZ31 s pomo~jo anodne oksidacije v alkalni raztopini NaOH-Na2SiO3. Potek in mehanizem nastanka nanodelcev so raziskovali in okarakterizirali z uporabo XRD, SEM in EDS. Dodatek nanodelcev ni spremenil osnovnega procesa anodizacije Mg zlitin. Debelina oksidnega filma se ujema s spremembo elektri~ne napetosti in~asa oksidacije. Prvih 5 sekund anodizacije se masa vzorcev manj{a, kar ka`e na to, da dominira proces raztapljanja Mg. Sledi nara{~anje mase vzorcev, kar pomeni, da pomembno vlogo v procesu predstavlja tvorba oksidnega filma. Nastali oksidni film vsebuje nanodelce Al2O3, absorbirane na povr{ini v Mg matrici. Nanodelci so vklju~eni v proces rasti oksidnega filma. Kompozitni oksidni film je sestavljen iz MgO, Mg2SiO4 in a-Al2O3. V procesu nastajanja oksidnega filma nanodelci Al2O3 zapolnijo pore in razpoke potem, ko se absorbirajo na povr{ino filma. Dodatno so lahko le-ti stalno ujeti v oksidnem filmu, kar oja~a njegovo strukturo in izbolj{a njegove lastnosti. Klju~ne besede: zlitina na osnovi magnezija, kompozitni oksidni film, nastanek Al2O3 delcev