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In this study, we conversed Bisbromomethyl-biphenyl and thiourea to a new biphenyl dithiol (BPDT) and used it as a co-curing agent. Its structure was confirmed by means of F-TIR, 1H-NMR, and MS spectra. We co-cured the conventional diglycidyl ether of Bisphenol A (DGEBA) epoxy resin with BPDT and aliphatic tetra-thiol (Pentaerythritol tetrakis(3-mercaptopropionate)) to get a colorless encapsulant (HRIL). The optical properties were measured after curing process. The refractive index was 1.64 which is the highest refractive index among the current existed LED encapsulant, and the transmittance of the cured HRIL was above 85% on the visible wavelength. Furthermore, the effect on light efficiency of LED was evaluated and found that the light efficiency was promoted 14%, compared with the conventional epoxy encapsulated LED. This result certainly reveals that the high refractive index encapsulant can effectively enhance the light efficiency of LED. We also analyzed the thermal mechanical property and thermal stability of the cured HRIL which contained a hindered amine light stabilizer (HALS) by TMA and TGA. The Tg of the cured HRIL was 85.2degC and the thermal decomposed temperature was 327.7degC. After 110degC/1000h thermal aging test, the decrease of transmittance was less than 5% and the thermal resistant result unveils that the novel high refractive index encapsulant is suitable for high performance LED applications.