The Gate-Magnetization method developed by Yokoi and coworkers for thermoplastics was used to investigate the flow behavior of glass-fiber reinforced phenolic resin compound inside a mold cavity. Plug now builds up in the cavity with a decrease of the local viscosity along the cavity wall due to the heat transfer from the wall and shear heating. Simultaneously, a high viscosity layer builds up from the crosslinking reaction along the cavity wall. The thickness of the high viscosity layer increased as the injection rate decreased. An unstable flow boundary formed between the reacted high viscosity layer along the wall and the low viscosity layer just under the layer.