Abstract: Near-infrared (NIR) light induced reverse atom transfer radical polymerization (ATRP) was achieved by using upconversion particle (UCP), which can convert the NIR light into light with ultraviolet-visible (UV-Vis) wavelength, as the internal light source, phenyl bis(2, 4, 6-trimethylbenzoyl)-phosphine oxide (BAPO) as the initiator, copper bromide (CuBr₂)/N, N, N', N", N"-pentamethyldiethylenetriamine (PMDETA) as the catalyst, methyl methacrylate (MMA) as the main monomer, N, N-dimethylformamide (DMF) as the solvent, and the reaction condition of c(MMA)∶c(BAPO)∶c(CuBr₂)∶c(PMDETA)=300∶1∶1∶3, V_MMA∶V_DMF=1∶1, and NIR laser power was 16.5 W/cm². The UV-Vis spectrum of BAPO, the emission spectrum of UCP, and the photolysis of BAPO under irradiation of NIR light assisted by UCP indicated adaptability of BAPO and UCP. The process and reverse ATRP mechanism of this system were investigated by adjusting the conditions of polymerization, including whether UCP, BAPO or catalyst existed or not. In addition, the energy source of this polymerization was investigated by simulating the temperature of this system under NIR light irradiation. The results confirmed the photoluminescence of UCP under irradiation of 980 nm NIR light and the polymerization was initiated by fluorescence instead of thermal effect. UCP-assisted NIR induced reverse ATRP was capable of polymerization of various types of monomers, including MMA, methyl acrylate (MA) and styrene (St). The kinetics of MMA polymerization was investigated under UCP-assisted NIR induced reverse ATRP and a linear semilogarithmic plot of monomer concentration versus polymerization time was observed. The living character of the polymerization was confirmed by both the linear tendency of molecular weight evolution with monomer conversion and a chain extension experiment. The UCP-assisted NIR induced reverse ATRP provided well-defined polymer with relatively low polydispersity and excellent chain-end fidelity.