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High temperature components with notches, defects and flaws can producecrack initiation and crack propagation under service conditions. Fracture mechanicsprocedures are needed to study crack problems and to support an advanced remnant lifeevaluation. Since a more flexible service mode of steam power plants causes a highernumber of start up and shut down events, creep-fatigue crack behaviour is decisive forlife assessment and integrity of components. Fracture mechanics experiments normallyare carried out under air conditions although in some cases internal defects are not incontact with air. Therefore, it is of interest to which degree environmental conditions, e.g.crack tip oxidation, can influence crack initiation and crack growth behaviour.In order to enlighten these problems for high temperature components, the crackinitiation time and crack growth rate were determined in air and inert gasenvironment on 1% CrMo(Ni)V- and 10% CrMoWVNbN-steels.Crack initiation and crack propagation under creep and creep-fatigue conditions havebeen described with the usual fracture mechanics parameters C*, KI and ÄKI and amodified "Two Criteria Approach" was applied. Small differences in crack initiationand crack growth were detected for the different environmental conditions.