Neutrinos are elementary particles in the Standard Model.There are three flavors of neutrinos, known as ν_e, ν_μand ν_τ . Theneutrino flavor states are superpositions of three masseigenstates (ν₁, ν₂ and ν₃), which undergo quantum interferenceduring the traveling, causing neutrino flavorchanges. This phenomenon is known as neutrino oscillation or neutrino mixing.The amplitudes of oscillation are connected to themixing angles θ₁₂, θ₂₃ and θ₁₃. The oscillation frequenciesare determined by the difference of squared neutrinomasses, Δm²_ij = m²_i-m²_j. 

　　θ₁₂ and Δm²₂₁ were measured by solar neutrino experiments and a reactor neutrino experiment KamLAND.θ₂₃ and |Δm²₃₂| were measured by atmospheric and accelerator neutrino experiments. The last unknown mixing angle θ₁₃ was determined to be unexpectedly large by the DayaBay Reactor Neutrino Experiment, as well as otherreactor neutrino experiments Double Chooz and RENO, and accelerator neutrino experiments T2K and MINOS. These measurements open a new era of neutrino experiments.The next generation neutrino oscillation experiments will focus on determining the sign of Δm²₃₂ (mass hierarchy), precisely measuring all oscillation parameters and searching for CP violation in the neutrino oscillation. 

　　The major goal of JUNO is determining neutrino mass hierarchy by precisely measuring the energy spectrum of reactor electron antineutrinos at a distanceof ~53 km from the reactors. A relative measurement technique was developed based on the interference of two different oscillation frequency components driven by Δm²₃₂ and Δm²₃₁, which can reach to a sensitivity of Δχ²>16 in the idea case of single reactor and single detector, and Δχ²>9 considering the spread of reactor cores and uncertainties of the detector response. If the absolute value of Δm²_μμ(which is an approximation of Δm²₃₂) measured from acceleratorexperimentsis included with a precision of 1%, the sensitivity of mass hierarchy can be improved toΔχ²>25 and Δχ²>16 in the idea and real case, respectively. 　 

　　                   Sensitivity of mass hierarchy                                   Precision of oscillation parameters 

　　JUNO is going to improve the precision of Δm²₂₁, Δm²₃₂ and sin²θ₁₂ to be better than 1%. Considering the precision of sin²θ₁₃ can be measured to ~4% by Daya Bay, the unitarity of the neutrino mixing matrix can be probed to 1% level. Besides, JUNO has other scientific possibilities such as supernova neutrinos, geo-neutrinos, solar neutrinos, atmospheric neutrinos, and exotic searches.