The enhanced sensitivity of up-conversion luminescence is imperative for the application of up-conversion nanoparticles (UCNPs). In this study, microfibers were fabricated after co-doping UCNPs with polymethylmethacrylate (PMMA) and silver (Ag) solutions. Transmission losses and sensitivities of UCNPs (tetrogonal-LiYF:Yb/Er) in the presence and absence of Ag were investigated. Sensitivity of up-conversion luminescence with Ag (LiYF:Yb/Er/Ag) is 0.0095 K and reduced to (LiYF:Yb/Er) 0.0065 K without Ag at 303 K under laser source (980 nm). The UCNP microfibers with Ag showed lower transmission losses and higher sensitivity than without Ag and could serve as promising candidate for optical applications. This is the first observation of Ag-doped microfiber via facile method.