Pursuing highly sensitive thermometry is a permanent goal in recent scientific research, which can be achieved through Fluorescence intensity ratio (FIR) technique due to its fast response, excellent accuracy, high spatial resolution and tunable size. Herein, highly sensitive micro-thermometers based on FIR of the thermal coupled levels (TCLs) H-2(11/2)/S-4(3/2) -> I-4(15/2) from Er3+ in phosphors NaLa(MoO4)(2): Yb3+/Er3+ up-conversion (UC) were developed, which emits dazzling green light with the excitation of 980 nm laser and its UC spectra consisted of intense green bands peaked at 530 and 552 nm and a negligible 658 nm red emission (F-4(9/2) -> I-4(15/2)), respectively. The maximum absolute sensitivity approached to 0.023 K-1 at 490 K through assessing FIR based on thermo-responsive green emissions. Results suggest the present green-emitting phosphors have great potential to serve as UC thermometers with high and stable sensitivity, which greatly meet the growing demand for sensing applications.