By using Yb3+ and Er3+ as double sensitizers and 980 nm laser as pump light, unusual radiative transitions H-5(3-7) --> F-7(0-3), L-5(6) --> F-7(0), D-5(3) --> F-7(2,3), and D-5(2) --> F-7(0,1,2,3) of Eu3+ ions in NaYF4 nanocrystals were observed. H-5(3-7), L-5(6), and D-5(2,3) levels of Eu3+ have been populated by two-step energy transfer process of Yb3+ --> Er3+ --> Eu3+ and become efficient radiative levels in the upconversion scheme. Experimental data indicate that when the excited Er3+ ions serve as donors, it is easier to transfer energy to acceptors after they are excited to the levels with longer lifetime. (C) 2009 Elsevier B. V. All rights reserved.