Sintering is one of the most important steps for powder metallurgy (PM). In order to observe the sintering process directly, a high temperature optical microscope (HTOM) was used to in-situ investigate the Al and Mg binary alloy system with a weight ratio of 9:1. A typical physical evolving character was found in this system. The melting process of magnesium particles is not synchronous, first from the particle border, then in the grain boundary, and last into the whole magnesium particle. When sintering temperature is above a critical temperature Tc, the magnesium particle melts in a very short time, and the sintering rate is accelerated with the increasing of the cold compact density. These experiment results afford a direct reference to the sintering routine optimization.