The strontium to calcium ratio of coral aragonite is mainly controlled by three factors: (1) the strontium distribution coefficient between aragonite and seawater, (2) the Sr²⁺/Ca²⁺ ratio of the surface oceanic water, and (3) biological effects. The mean oceanic residence time of strontium and calcium are calculated to be 4.9×10⁶ years and 1.0×10⁶ years, respectively. Therefore, the Sr/Ca ratio in seawater is considered to be quite uniform throughout the world's surface seawater. If biological effects are considered to be negligible, Sr/Ca ratio of coral aragonite can vary as a simple function of temperature in which corals have grown. Steve Smith has clearly demonstrated, from a series of careful macrocosm experiments on Pocillopora damicornis, that a relationship exists between Sr/Ca in coral aragonite and ambient seawater temperature. The Sr/Ca ratio in coral aragonite is shown as a simple linear equation of ambient seawater temperature, classified of the "Sr/Ca thermometry". Recent high precision data of Sr/Ca analysed by mass-spectrometer, further calibrated the Sr/Ca thermometer using Porites corals. But recent studies also pointed out spatial variations (〜1%) of surface seawater Sr/Ca. In addition, biological effects such as taxonomy and growth rate are discussed in this paper. Re-evaluation of above three factors will provide a more reliable coral Sr/Ca thermometer.