Sensing temperature with high precision and high spatial resolution is challenging and requires novel temperature measurement techniques. Recently, an atomic-scale thermal sensor based on a defect center in diamond, i.e., a nitrogen-vacancy (NV) center, has been developed, and successfully demonstrated temperature sensing at the mK level and a few tens of nanometers. Here we discuss a temperature sensing mechanism based on the NV center in both experimental and theoretical aspects. At room temperature, we show temperature sensing over a wide-range of temperatures similar to 90 K with a precision of 0.2 K. We also map temperature gradients in a bridge-like device a few hundreds of micrometers long. In addition, we theoretically compare three sensing protocols and analyze temperature sensitivity to find optimal measurement time and NV concentration for the ensemble measurement.