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NaGd(WO 4 ) 2 :Yb 3+ /Er 3+ (NGW:Yb 3+ /Er 3+ ) phosphors codoped with Yb 3+ (12-24at.%, at.% i.e. atomic percent) and Er 3+ ions (1-10at.%) were synthesized by a facile hydrothermal process. Under 980 nm excitation, NGW:Yb 3+ /Er 3+ phosphors exhibited strong green upconversion (UC) emission bands centered at 532 nm ( 2 H 11/2 → 4 I 15/2 ) and 554 nm ( 4 S 3/2 → 4 I 15/2 ) and weak red emissions near 658 and 671 nm ( 4 F 9/2 → 4 I 15/2 ).The optimum doping concentrations of Er 3+ and Yb 3+ for the highest emission intensity were determined by using X-ray diffraction (XRD) and photoluminescence (PL) analyses. Concentration dependent studies revealed that the optimal composition was realized for the 20 at%Yb 3+ and 6.0 at% Er 3+ -doping concentration with a strong green emission. A possible UC mechanism for NGW:Yb 3+ /Er 3+ depends on the pump power is discussed. The temperature dependence of the fluorescence intensity ratios (FIR) for the two green UC emission bands peaked at 532 and 554 nm was studied in the range of 293-573 K under excitation by a 980 diode laser and the maximum sensitivity was approximately 114.9610 -4 K -1 at 453 K. This indicates that NGW:Yb 3+ /Er 3+ phosphors are potential candidates for optical temperature sensors with high sensitivity. NaGd(WO 4 ) 2 is a member of the alkali rare earth tungstates (ARE(WO 4 ) 2 , where A=alkali metal ions, RE=rare earth ions) family. It is considered to be a promising luminescent host lattice because of its good chemical durability and excellent physical properties as well as the relatively low lattice phonon energy. Recently, several chemical synthesis techniques to prepare RE doped NGW phosphors are available such as high temperature solid-state reaction 18 , sol-gel method 19 , hydrothermal method 20 . Hydrothermal method is better over other conventional techniques in terms of its better homogeneity, low cost, low processing temperature, less energy consumption and uniformity in particle size. It was reported that strong luminescence properties can be observed in RE (such as Pr 3+ , Sm 3+ , Eu 3+ and Dy 3+ ) doped NGW phosphors 18-21 . However, to the best of our knowledge, there is little or no investigation on the UC emission and temperature sensing performances of NGW:Yb 3+ /Er 3+ microcrystals prepared by hydrothermal method. We report here the hydrothermal synthesis of the Yb 3+ /Er 3+ co-doped NGW microcrystals. The luminescence properties of NGW:Yb 3+ /Er 3+ microcrystals are investigated by changing doping concentrations of Yb 3+ (and Er 3+ ) ions in the host. Additionally, their thermometry behaviors have also been illustrated by FIR technique. Experimental Sample preparationAll the chemicals of Gd 2 O 3 (99.99%), Er 2 O 3 (99.99%), Yb 2 O 3 (99.99%) Na 2 WO 4 2H 2 O (AR), oleic acid (AR), HNO 3 and NaOH were used as the starting materials without any further purification. NGW:Yb 3+ /Er 3+ samples for different Yb 3+