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Rare-earth doped upconversion(UC)phosphors,which can convert a low energy photon into a high energy photon through two-or multi-photon processes,have been attracted considerable attention for their potential applications in many fields,such as solid-state lasers,biological detection,photothermal therapy,temperature sensors,etc.In this paper,Er3+/Yb3+ co-doped -NaYF4 nanocrystals were synthesized through microwave-assisted hydrothermal process.It can be seen from the field emission scanning electron microscopy(FE-SEM)image that the sample exhibits irregular spherical morphology and the size is uneven about 75 nm.Temperature-dependent upconversion luminescence(UCL)and sensing properties were studied.It is interesting to observe that with the increase of temperature(303-573 K),the UCL intensity does not quench monotonously but reach a minimum around 483 K and then enhances.However,the UCL intensity quenches monotonously with the following reduced temperature(573-303 K),namely cooling process.The intensity ratio of 2H11/2→4I15/2 to 4S3/2→4I15/2 at any measurement temperature point keeps almost consistent in all heating and cooling processes,indicating the consistency of temperature in each spectrum measurement at all temperature points regardless of the heating or the cooling process.The anomalous temperature-dependent UCL and temperature sensing properties were investigated.