The phenomena of dispersion of filler in polymer matrix should be understood accurately to develop polymer composites. We reviewed the dispersion mechanisms and dominant factors about filler dispersion. The dispersion mechanisms are divided into two main types, rupture and erosion. These are essentially the same breakdown phenomenon, that is removing two particles until the distance interparticle force is not effected. Also for higher dispersion, the dominant phenomenon is erosion. There are two important factors for dispersion, stress and energy. The stress exceeding cohesive force of filler is necessary to initiate dispersion, and energy is necessary to advance it. These two factors should be considered concurrently when one studies filler dispersion. These two factors are used by modifying for appropriate form in many investigations. Specific energy is an effective index to evaluate the degree of dispersion. However, it is necessary to postulate the following conditions if we use it. First, it must be under the critical stress which is equal to tensile strength of filler. Moreover, the provided energy must be dissipated for dispersion. Numerical flow simulation technique is a useful way to estimate the dispersion index more accurately.