Cyclone separators are widely used to eliminate particles flowing through pipelines in equipment from various industrial processes. Unlike general filters, cyclone separators can constantly and effectively eliminate foreign substances present in the fluid flowing through the equipment. In this study, we fabricated mini-hydrocyclone separators using the 3D printing method for application in the steam and water analysis system (SWAS) in a thermal power plant instead of the conventional strainer filter. The gravimetric method was used to measure the separation efficiency of the hydrocyclone separators and compare the weights of the sludge discharged from the underflow and overflow outlets. The outlet flow ratio was optimized by adjusting the diameters of the spigot and vortex finder of the separators, which influenced the outlet flow rate. To apply the gravimetric method more objectively, the optimum values of the diameters of the vortex finder and spigot with an outlet flow ratio of 1 were determined using full factorial design (FFD) in the design of experiments (DOE). The obtained values were verified through numerical analysis using the ANSYS Fluent software. Furthermore, after fabrication of the mini-hydrocyclone separators using an SLA-type 3D printer, we conducted a numerical analysis, and the results were compared with that of the actual experiment. It was observed that the use of FFD can effectively optimize the desired outlet flow ratio in the mini-hydrocyclone separator. In addition, the changes in the outlet flow ratio do not affect the separation efficiency of the cyclone separators.