With the method of high-tempreture thermal decomposition, Li:YF4:Er,Yb nanocrystals with an average diameter of about 10 nm are synthetized, which are doped into SU-8 and used as the active layer of the waveguide amplifier. The waveguide amplifier based on SU-8 doped with Li:YF4:Er,Yb nanocrystals is fabricated by using SiO2 as an upper cladding and P(MMA-GMA) as a lower cladding. For an input signal power of about 0.05 mW and a 980 nm pump power of about 180 mW, a relative optical gain of about 2.3 dB/cm at 1535 nm is achieved in polymer waveguide doped with Li:YF4:Er,Yb nanocrystals.