Based on theoretical analysis, a new general equation of mean particle size applicable to both conventional atomization methods and new atomization processes is presented. In gas atomization, it is equivalent with and can be changed into Lubanska Equation. In centrifugal atomization, it can be changed into the equations that are currently the most widely used. In water atomization, it is similar to the equation proposed by Grandzol and Tallmadge. According to the universal equation, new correlations for mean particle size in novel atomization processes such as Hybrid Atomization and Multistage Atomization were proposed and agreed with our experimental data well.