This study investigated the characterization of the additive powder on microstructural evolution during the heating of the powder mixture of additive and filler metal powder by in-situ high temperature environmental scanning electron microscopy (HT-ESEM) up to 1200°C. The IN738 powder (additive) and BNi-3 powder (filler metal) were used for wide-gap brazing process. A field emission gun environmental scanning electron microscope (XL 30 ESEM-FEG, FEI) equipped with a 1500°C hot stage was used for in-situ gaseous secondary electron imaging at high temperature (HT-ESEM image). The melting of filler metal powder initiated at 1224K and was spread on the IN738 additive powder with increasing temperature. After cooling, the IN738 additive powder was increased from 75μm to 100μm. It was found that the additive powder added to the wide-gap brazed region avoided the possibility of directional solidification.