NaYF4:Yb3+,Er3+@Ag core-shell nanocomposites were prepared. Due to the outer shell of Ag, some modifications were found on the upconversion (UC) processes of NaYF4:Yb3+,Er3+ core nanoparticles (NPs) caused by the co-interaction of surface effect, surface plasma absorption effect and local thermal effect under infrared radiation. Upon 980 nm excitation, the relative UC intensity of the green (H-2(11/2), S-4(3/2)-I-4(15/2)) to red (F-4(9/2)-I-4(15/2)) and the slope of power-dependence increased intensively, and three-photon population process for the green level appeared. (C) 2012 Elsevier B.V. All rights reserved.