It has been previously confirmed that the dissolution of magnesium hydroxide was a crucial procedure in its application process. Note that the ultrasound is an effective method for enhancing solid dissolution. In this study, the enhancement of ultrasound on the dissolution of magnesium hydroxide was investigated by the pH-stat method. The titrating results indicated that the promotion effect of ultrasound was only be observed at some certain cases, such as lower pH value. Meanwhile, the activation energy data obtained based on Shrinking Core Model for sonication case ranged from 6.56 to 49.13 kJ/mol, implying that magnesium hydroxide dissolution proceeds at sonication case cannot be explained well by this model. Nanoscale observation was conducted to identify the crystal surface variations during the dissolution process by using SEM and AFM. The analysis results indicated that the dissolution behaviors of magnesium hydroxide in the absence and presence of ultrasound were quite different. In the silent case, the dissolution followed the famous Stepwave model. As for the sonication case, the larger particle was broken into flake by the ultrasound firstly. Then, the heterogeneous global dissolution accompanied by the Stepwave model drove the dissolution process. The analysis results of nanoscale observation provided a reasonable explain for the kinetics research results on micro-level.