Luminescence phenomenon from NH2-terminated third generation poly(propylene imine) dendrimers in water was investigated. Two excitation bands (340 and 370 nm) and corresponding emission bands (∼400 and ∼450 nm, respectively) were detected. Emission bands were intensified with time, heating and air-bubbling and converged at certain intensities, where the ∼400 nm band was red-shifted. The initially weak emission band at ∼450 nm in an alkaline solution was intensified up to the comparable intensity to the strongest emission band at ∼400 nm in an acidic solution, after long time. While 1H NMR spectra depended on pH owing to the protonation of amine, those were slightly affected by the time, heating and air-bubbling. However, no new 1H NMR signals appeared even after long time, indicating no finding of chemical structure change of dendrimer. This suggests that the luminescent property should result from any interaction of amine with oxygen but not the chemical reaction.