A parametric study on the influence of a pre-oxide layer on the high temperature steam oxidation behaviour of Zircaloy-4 cladding has been conducted at the Institut de Radioprotection et de Sûreté Nucléaire (IRSN). Zy4 cladding samples were pre-oxidised in pure oxygen to obtain pre-oxide thicknesses ranging from 10 to 70 μm. Samples were oxidised at 900 °C for 6000 s in steam and then were characterised by metallographic examinations, Vickers micro-hardness measurements, Ring Compression Tests, global hydrogen analysis, EPMA oxygen profiling and mapping. All pre-oxidised sample present large hydrogen pickup and high oxygen concentration in the α+β phase. The mechanical behaviour is degraded compared to initially as-received samples whatever the pre-oxide thickness after HT oxidation.