Up-conversion luminescence have been studied on Yb3+-Er3+ co-doped fluoride samples. Two infrared lasers with wavelength of 930 nm and 858 nm are carefully chosen as excitation sources. The experimental results suggest direct cooperation sensitization up-conversion rather than two-step sensitization up-conversion is responsible for the increased population of 2H11/2 (Er3+) and thus the increased green emission in the region 514～574 nm in Yb3+-Er3+ co-doped system.