Synthesis of size tunable nanomaterials creates distinct chemo-physical properties. Recently, the popularity of magnetic iron oxide and cerium oxide (CeO2) nanocrystals enables researchers to use magnetic iron oxides (magnetite and ferrites) in size dependent magnetic separation and CeO2 as an automobile exhaust gas catalyst. This research shows production of diameter-controlled monodisperse magnetic iron oxide (ranging from 3 to 40 nm in diameter) and CeO2 (from 3 to 10 nm in diameter) nanocrystals with exceptional narrow diameter distribution (σ<10%). The morphology and composition of the nanocrystals were varied by use of diverse metal precursors, reaction temperature, time, cosurfactants, and molar ratio between metal salt and surfactant. Now the narrow diameter distributions of preformed magnetic iron oxide nanocrystals made it possible to grow diameter controlled uniform CNTs. The correlation between aluminum ferrite nanocrystal diameter and CNT diameter was nearly one. Additionally, we could synthesize the highest percentage (60%) of single walled CNTs from the smallest aluminum ferrite nanocrystals (4.0 nm). Because of the synthesis of uniform nanocrystalline CeO2, we could study diameter dependent antioxidant properties of nanocrystalline CeO2; antioxidant capacity of CeO2 was nine times higher than a known commercial standard antioxidant, Trolox. In addition, the smallest CeO2 nanocrystal (4 nm) decreased the oxidative stress of human dermal fibroblasts (HDF) exposed to hydrogen peroxide. These works suggest better understanding of monodisperse nanocrystal synthetic mechanism and potential uses of the materials, such as high quality CNT growth using magnetic iron oxides as precursor catalysts and the reduction of oxidative stress in cells using monodisperse CeO2 nanocrystal as an antioxidant for reactive oxygen species in biological media.