Measurement of Mg/Ca ratio of foraminifera shell is the most popular means in calculating seawater paleotemperature now. It is applicable in low and high latitude areas, to determine surface and deep seawater temperatures. The standard error is 1.1℃, and when the effects of salinity and pH are considered, the error increases to ±1.3℃. The tropical sea surface temperature obtained from Mg/Ca ratio appears to lead ice sheet demise by ~3 ka, which suggests a prominent role of the tropics in pacing ice age cycles. Combining the oxygen-isotope paleotemperature equation, the Mg/Ca ratio of foraminifera shell yields the δ18O water at the time of shell precipitation and the changes of the global ice sheets. This approach has become a good thermometer in determining paleotemperature variations in paleo-environmental studies.