Behaviour of materials under repeated application of loads of varying magnitude is an important problem in design aspect. Most structures and machine parts "which are made of aluminium - magnesium alloy " are subjected to more or less random fluctuations of loads. Therefore, the main purpose of this work is to obtain data which can be used as a guide for predicting the behaviour of aluminium - magnesium alloy in service. Presented here are the results of endurance fatigue tests for the material. The specimens have been subjected to rotating bending stress, and also the staircase testing procedure was followed to determine the endurance strength of the material. Two-step tests for cumulative fatigue damage were conducted to the material. A comparison between the experimental results with the cumulative fatigue damage rules is made. An agreement for the isodamage rule with the experimental results. The experimental results indicated that the amount of damage is more than unity for low-high sequences and less than unity for high-low sequences with the same different of magnitude of damage than unity.