Analyses of cooperative upconversion were performed for two kinds of bismuth oxide-based erbium-doped fibers (BIEDFs) with and without lanthanum (La) and boron (B). The pump energy dependences of the fluorescence lifetimes of Er3+: 1.55 mu m lateral emissions were investigated to estimate cooperative upconversion (CUP) coefficients. The CUP coefficients increased with Er concentrations in all the BIEDFs, whereas the coefficients in the La/B co-doped BIEDFs were smaller than those in the La/B-free BIEDFs. It is strongly suggested that the gain improvement in La/B co-doped BIEDFs is attributed to the suppression of CUP.