The aim of the present study was to investigate age hardening properties of the AZ91 Mg alloy. Rockwell hardness test, energy dispersive X-ray spectroscopy, optical microscopy and scanning electron microscopy were performed to identify the age hardening behaviors. The degree of age hardening of AZ91 alloys was changed with the time and temperature of the artificial aging treatment. Higher aging temperature shortened initial aging time and time for reaching the peak hardness, while too high temperature reduced the amount of age hardening. So it is necessary to choose suitable aging temperature and time. It was found that precipitated Mg-Al compound was the age strengthing phase in AZ91 Mg alloy. And hardness increased abruptly at the beginning of intragranular precipitation after intergranular precipitation was nearly saturated.