This presentation reports our recent studies on the understanding of spin-dependent processes in TADF (Thermally Activated Delayed Fluorescence) light-emitting materials based on magneto-optical studies. Recently, we have performed magneto-optical studies on TADF light-emitting molecules (DMAC-TRZ) by using magneto-photoluminescence (magneto-PL). Our magneto-PL studies provide the first evidence that the TADF is a spin-dependent process occurring in charge-transfer states. Essentially, the key spin-dependent process, namely spin mixing, necessarily required to activate the TADF, is determined by the competition between two critical parameters: (i) exchange interaction which functions as a resistance force to the TADF and (ii) spin-orbital coupling which acts as a driving force to the TADF. Therefore, controlling the exchange interaction and spin-orbital coupling becomes a critical issue in the development of highly efficient TADF light-emitting materials. By using magneto-PL studies, we further found that, doping soluble magnetic nanoparticles (surface-modified Fe3O4) can conveniently change the exchange interaction and spin-orbital coupling, and consequently alters the TADF rate. At low doping concentrations, the spin-orbital coupling is enhanced, leading to an increase on TADF rate. However, at high doping concentrations, the exchange interaction is increased, causing a decrease on the TADF rate. Furthermore, we studied the polarization effects of spin mixing in liquid TADF materials by using various solvents with different polarities. We observed that increasing the host polarization can directly weaken the spin mixing and leads to a decrease on the TADF rate. This experimental observation indicates that host polarization can weaken the spin-orbital coupling and thus decreases the driving force to TADF. Clearly, the magneto-PL studies provide an insightful understanding on the spin-dependent process to control the TADF rate in organic light-emitting materials.