Laser gain(λ=632.8 nm, transition 3S2-2P4)of 3He-Ne laser system (PHe:PNe=5:1,Ptotal=4×104 Pa) radiated by thermal neutron from steady operation reactor is measured. The measured gain coefficient is 1.7×10-2/em at thermal nuetron flux of 2×1012 n/cm2-s-Gain curve vs flux of thermal neutron is investigated. The influence of temperature on gain is discussed. The test shows the possibility of lasing from nuclear-pumped 3He- Ne laser system.