Acoustic Emission (AE) is one of the condition monitoring and diagnosing techniques of rotating machine elements such as gears, bearings, etc. So far, many studies about fault diagnosis on gearboxes have been implemented for vibration monitoring. In addition, a great deal of research on spur gears has been done for understanding the possible gear faults by considering their acoustic characteristics. In this study, possible faults in gears were analysed by the AE technique. A single-stage gearbox system comprising both helical and spur gears was used to identify the existence of possible gear faults, such as pitting and cracking at the tooth root. Noise signal in time-domain is converted to frequency-domain by using Fast Fourier Transform (FFT). In the experimental stage, artificial faults were implemented, and some mathematical parameters such as Root Mean Square error (RMS), Crest Factor (CF), and maximum value of noise level were considered to identify the fault occurrence at the meshing gear. The results show that the AE technique is very effective in diagnosing the defects in a gear system by a contactless measurement. Also, compared to the other diagnostic approaches, it is clear that the gear defects can be determined at an earlier stage by the AE technique.