In molybdenum (Mo 6+ ) doped upconversion (UC) materials, such as Yb 3 Al 5 O 12 and Yb 2 Ti 2 O 7 , anomalous UC luminescence has been discovered recently. In this paper, Yb 2.75−x Er 0.25 Mo x Al 5 O 12 phosphors with different Mo-content were synthesized via the sol-gel method. According to the analysis of the structural components and the luminescence spectra of the samples, we found that the Yb 2 (MoO 4 ) 3 impurity which was introduced into YbAG sample by Mo 6+ doping is responsible for the anomalous intense UC luminescence. Quantitive analyses on the dependence of UC luminescence intensity upon pump power indicate UC green luminescence is a two-photon process for Yb 2 (MoO 4 ) 3 : Er 3+ 1% and Yb 2.25 Er 0.25 Mo 0.5 Al 5 O 12 systems, which is far less than the lowest excited state level of MoO 4 2− .