The clusters of Eu^(3+) ion in Eu(DBM)3Phen-doped polymethyl methacrylate (PMMA) have been studied by three means. The relative fluorescence intensity ratio of the 5D0 -> 7F2 to 5D0 -> 7F1 transitions with different concentrations of Eu^(3+) in Eu(DBM)3Phen-doped PMMA and metastable-state (5D0) lifetime dependence on Eu^(3+) concentration are analyzed. The analysis indicates that there are no clustering effects in Eu^(3+) ions of Eu(DBM)3Phen-doped PMMA when the Eu^(3+) doping concentration is up to 1.0 wt.-%. At the same time, the clustering effect has not been observed by the scanning near-field optical microscopy (SNOM) in Eu(DBM)3Phen-doped PMMA with 1.0 wt.-% of Eu^(3+) ions. The analysis reveals that a high concentration of Eu^(3+) can be incorporated into polymer optical fiber (POF) without clustering effect.