It is demonstrated here that relative sensitivity should gain more concern than absolute sensitivity when designing a new type of nano-thermometer based on luminescence ratiometric technology. Following the NIR excitation by a 980 nm laser diode, the 520/527 and 540/547 nm upconversion luminescence bands, which are respectively ascribed to the H-2(11/2)-I-4(15/2) and S-4(3/2)-I-4(15/2) transitions of Er3+ ions embedded in NaYF4 nano-crystals, are investigated as a function of temperature between 303 and 483 K. It is found that the 520/540, 520/547, 527/540 and 527/547 nm emission lines can constitute four pairs of thermally coupled energy levels. These four pairs of lines own the different absolute sensitivities. However, the relative sensitivities for them are identical at the same temperature. A third parameter, that is, temperature resolution, is then introduced to identify the difference between the two sensitivities. It is found that these four pairs of lines own the same temperature resolution, suggesting that absolute sensitivity is an intermediate parameter and relative sensitivity reflects the fundamental nature of sensors.