An experimental study to understand the characteristics related to chatter occurrence in micro-milling operations is presented. Accelerometers are used to measure the vibration signals in the machining process. The acceleration signals are then analysed in the time domain and the frequency domain. Along with the onset of chatter, it is found that there is a characteristic shift of the dominant frequency components in addition to the change of vibration amplitude. A modulation of the spindle frequency around the chatter frequency is also found to be present in the vibration signal. A dimensionless chatter indicator based on revolution root-mean-square values is designed and used to evaluate the stability of the micro-milling process. It is shown that the proposed indicator provides a simple, but effective way to detect chatter onset.