In this work, the growth mechanism and adhesion of plasma electrolytic oxidation (PEO) coatings on 2024Al alloy were studied under different cathodic voltages. It is found that the growth of PEO coatings is characterised by both the inward growth and outward growth, which result from the oxidation of the substrate and the deposition of electrolyte-borne compounds. Moreover, the growth mechanism and adhesion of PEO coatings are closely related and significantly influenced by the applied cathodic voltages. When the cathodic voltage increases, the oxidation of the substrate becomes more predominant, causing more inward growth. And the increase of inward growth results in more compact structure and higher adhesion of PEO coatings.