In case of GMAW-P application, the product quality is strongly dependent on the arc plasma performances. Therefore, in order to improve the application, it is important to understand and characterize the fundamental plasma properties. In this paper, different techniques (high speed camera , optical emission spectroscopy system, electrical data acquisition system, synchronous triggering device) were used to characterize the arc characteristic. The metal diffusion process from the peak time to the base time is discussed, as well as its physical characteristics. The results show that, in the peak time phase, a high concentration of metal vapor arc is concentrated in the plasma center of the range of about 1 mm, during the transition from the peak to the base pulse periods, the diffusion leads to an iron vapor flux from regions in the arc core to the arc fringes. The iron mol fraction of the peak time was up to 75%, only taking up 35% of the base time.