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In a previous work by the authors, the torsional fatigue of micronotched - cold drawntubes had been analized. Specimens with longitudinal shallow notches showed adominant mode III propagation ( throughout the tube thickness) even if mode I (at thedefect edges) was present. Indeed, mode II crack propagation along the tube axis wasobserved in the second propagation stage.In this work, the reason for this peculiar fatigue crack propagation is furtherinvestigated. In particular, the relationship between some ‘equivalent’ stress intensityfactors proposed in the literature and crack growth rates is analysed. With this aim,alternating torsional crack propagation tests have been carried out on notched tubularspecimens, being the trough-thickness notches both in longitudinal and transversedirection, in order to analyse the possible influence of the material anisotropy.Mixed mode and pure mode II crack paths have been observed. The crack paths wereanalysed through FE analysis and the relationship between actual stress intensity factorand growth rates is discussed.