The development of spherical amalgam alloy has given rise to a great deal of interest in the structure of set amalgam and physical properties prepared from these alloys. The present paper is concerned with the microstructure and amalgamation process of spherical amalgam alloy by scanning electron microscope. Major results were as follows : 1. The surface structure of spherical amalgam alloy shows petal-like structure formed by the growth of dendrites. This dendrites become larger as particle sizes increase. Such surface microstructure of spherical amalgam alloys makes the surface area larger inconceivably, thereby are regarded as a morphological factor to accerelate amalgamation. 2. Adjoinig areas of the dendrite branches and of the grain boundaries are smooth in annealing spherical alloys, but creasy and rough in non-annealing. 3. Commercial spherical alloys have minute particles on the surface. Some of the such particles are regarded as tin-rich segregation. Non-annealing amalgam alloys scarcely have such minute particles. 4. Some changes arise already 1 minute Hg-treatment on the alloy surface. This surface changes are divided broadly into two structures ; broad, irregular massive structure and net-work structure formed by the assembly of minute crystalites. It is assumed that the former is Ag-Hg phase and the latter is Sn-Hg phase.