Nowadays, the technology intends to use materials such as magnesium alloys due to their high strength to weight ratio in engine components. As usual, engine cylinder heads and blocks has made of various types of cast irons and aluminum alloys. However, magnesium alloys has physical and mechanical properties near to aluminum alloys and reduce the weight up to 40 percents. In this article, a new low cycle fatigue lifetime prediction model is presented for a magnesium alloy based on energy approach and to obtain this objective, the results of low cycle fatigue tests on magnesium specimens are used. The presented model has lower material constants in comparison to other criteria and also has proper accuracy; because in energy approaches, a plastic work-lifetime relation is used where the plastic work is the multiple of stress and plastic strain. According to cyclic softening behaviors of magnesium and aluminum alloys, plastic strain energy can be proper selection, because of being constant the product value of stress and plastic strain during fatigue loadings. In addition, the effect of mean stress is applied to the low cycle fatigue lifetime prediction model by using a correction factor. The results of presented models show proper conformation to experimental results پرونده مقاله