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Critical plane parameters evaluate fatigue damage on selected planes. These planes are expected to be aligned with the physical cracking planes. This paper examines the predictions of both fatigue life and crack initiation plane using two critical plane parameters: Fatemi-Socie (FS) and Smith-Watson-Topper (SWT). Tubular specimens were machined from AZ31B magnesium extruded sections. Cyclic tests were performed under multiaxial proportional and nonproportional loading conditions. Fatigue crack length and angle were measured using optical microscope. This investigation shows that even though fatigue life can be well predicted the critical plane assumption is not consistent with the observed cracking plane.