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Fracture processes of short glass fibre-reinforced thermoplastic tensile specimens have been investigated in relation to acoustic emission (AE) characteristics. Two fibre diameters (d = 10, 23 μm) were adopted for this study. Frequency analysis employing band-pass filters suggested that AE with higher amplitude was due to fibre breakages. Fibre–matrix interfacial failure and matrix fracture emitted lower amplitude AE waves. Reflected and transmitted optical microscopy, in combination with scanning electron microscopy, revealed that fibre breakage ahead of the artificial notch tip led to the initiation of a macroscopic fracture. This coincided with the results that AE peak amplitudes, Vp, showed a minimum at fracture initiation. When the macroscopic fracture propagated, Vp again increased rapidly, indicating additional fibre breakage. These fracture mechanism worked consistently for both d = 10 and 23 μm. Based on the above findings, AE source characterization was proposed for the stable fracture process of short fibre-reinforced plastics.