The photoluminescence of lanthanide ions inside fullerenes is usually very weak due to the quenching effect of the fullerene cage. In the case of Er@C-82, the near-infrared emission from the Er3+ ion is completely quenched by the C-82 fullerene cage. It remains challenging to turn on the photoluminescence of Er@C-82 and other monometallofullerenes. In this work, we adopt a covalent modification strategy to alter the electronic structure of the fullerene cage for sensitizing the near-infrared emission of Er3+ ions in metallofullerenes Er@C-2n (2n = 72, 76, and 82). After covalent modification with trifluoromethyl, phenyl, or dichlorophenyl groups, the erbium metallofullerenes exhibit photoluminescence at 1.5 mu m, which is the characteristic emission of the Er3+ ion. Particularly, the otherwise nonfluorescent metallofullerene Er@C-82 is transformed into fluorescent derivatives by using this strategy. The photoluminescence from the Er3+ ion is ascribed to energy transfer from the fullerene cage to the Er3+ ion. According to theoretical calculations, the sensitization the Er3+ ion by the fullerene cage is associated with the large HOMO-LUMO gap and the closed-shell electronic structure the metallofullerene derivatives. This work provides useful guidance for the design and synthesis of new fluorescent metallofullerenes.