Breakdown mechanisms of O/W and W/O nano-emulsions generated by a high pressure wet-type jet mill and a motionless mixer named Ramond Supermixer^® were proposed in the present research. Droplet size distribution, geometric standard deviation of the droplet size distribution (σ_g), zeta-potential, Sauter mean diameter and emulsion viscosity were investigated under the various combinations of operating variables such as dispersed phase and continuous phase viscosities, surfactant concentration, stabilizer concentration, dispersed phase mass fraction, milling pressure, flow rate etc. Empirical correlations were constructed for d₃₂ and σ_g, and a larger similarity was found to exist among the correlations. A mechanistic model for the droplet breakage was constructed by balancing the turbulent disruptive stress with the internal viscosity stress due to the internal viscosity of droplet and the surface cohesive stress due to the interfacial tension, for the viscous sub-range and interfacial sub-range, separately. Both models were satisfactorily fitted for O/W and W/O nano emulsions, regardless of the production process. The effect of stabilization after breakage of droplets, were also discussed. Since the proposed mechanisms are independent of the emulsifier and the type of materials, those can be universally used in any emulsifier to form nano-emulsions.