Spiking representing a periodic melting and solidification in the depth of fusion zone during high-intensity beam welding is experimentally and theorectically investigated in this work . A spike is a sudden increase in penetration beyond what might be called the average penetration line. Many spikes have voids in their lower portions because molten metal does not fuse to the sides of the hole, producing a condition similar to a cold shut in a casting. These defects seriously reduce the strength of the joint. Due to the significant role of specular reflection on absorption, an investigation of the beam characteristics, especially the focal location, on spiking is important. Furthermore, as the cavity base oscillates upward and downward relatively from the focal location, a central region subject to direct irradiation changes instantaneously from maximum to zero and vice versa. This leads to several hundred time difference in energy absorption and strongly periodic melting at the cavity base. Physical phenomenon of spiking is obtained by comparing between the measured and predicted data based on scale anlaysis of transport process near the cavity base and energy absorption as a function of focal location.