Like synthetic fiber reinforced composites, natural fiber reinforced composites possess a good potential to be used in high performance applications due to their good balance of mechanical and damping properties. Composite materials used in sporting goods equipment and automotive applications are subjected to repeating, regular loads. Therefore a clear understanding about the reliability of composite materials under fatigue/cyclic loading is important for their design in high performance applications. Currently, the fatigue performance of natural fiber reinforced composites are not well understood or characterized. The fatigue damage of flax fiber reinforced polymer matrix composites can be divided into two components: thermal damage due to self-heating in the sample and micro-mechanical damage due to damage creation (i.e. crack initiation, crack propagation, delamination, etc.). In this study, fatigue tests were conducted at four different loading frequencies and the two energy components defined were separated experimentally. The fatigue life of flax fiber reinforced composites was found to decrease with increasing loading frequency. Thermal damage due to the high self-heating temperature of the sample was found to be the main responsible form of energy which decreases fatigue life with increasing loading frequency. Micro-mechanical damage due to cyclic loading was not found to change significantly with increasing loading frequency.