Calcium sulfate nanorods (CS-NRs) are valuable materials utilized in various applications, particularly in the medical field. In this work, the size-controlled synthesis of CS-NRs was carried out on the basis of the micelle-mediated phase separation phenomenon. A nonionic surfactant, Triton X-114, was employed for the thermoresponsive phase separation of a homogeneous solution to a surfactant-rich phase. Whereas each specific ion, Ca2+ and SO42-, was difficult to individually extract when present at concentrations less than their equilibrium concentration (solubility product constant, K-sp), the synthesized CS microrods (CS-mu Rs) were extracted into the surfactant-rich phase (enrichment factor = ca. 50). The presence of nitric acid increased the size of the materials up to 6707 +/- 3488 nm on the long side and 87 +/- 37 nm on the short side. The addition of silver nanoparticles (Ag-NPs) to the reaction mixture led to the formation of much smaller products, i.e., uniform CS-NRs whose sizes were in the range of 89 +/- 15 nm (long side) and 25 +/- 4 nm (short side). The size of the extracted Ag-NPs and CS-NRs decreased with an increase in added Ag-NP concentration until their microscopic observation became difficult. The factors (such as additive concentration, pH, temperature) affecting size control were evaluated.