Vibrating the slope during cooling slope casting of a metallic alloy has shown helpful for causing the microstructure more spherical and thinner in a previous research by the present authors. This paper gives results from a direct observation of the SSM microstructure formation during cooling slope casting of an NH4Cl-H2O alloy with vibrating the slope. It is found that the vibration exerted to the slope can break down the solidification shell formed at the surface of the slope through rapid chilling and produce effectively fine globular grains in the microstructures. The main factors affecting the fine grain formation include the chilling extent, frequency and amplitude of the vibration of the slope, and the superheat of the alloy melt, respectively. The higher vibration frequency and amplitude, the lower superheat of the alloy melt, and the lower chilling temperature will help the formation of finer globular grains.