Acoustic Emission (AE) technique is employed to detect degradation events inside of lithium-ion batteries. A commercial 18650 type battery and a model half cell (Li/organic electrolyte /LiCoO2) are monitored with AE technique during charge and discharge cycles. In the commercial battery, AE events are detected during every discharge process at SOC of around 35% and 15%. These AE can be attributed to the fracture of cathode material by phase transition. In the model half cell, AE events are found early cycles in charge process owing to the SEI formation, however in discharge process detected only under high C rate.