In the Rb-Sr dating system, ages are calculated from tie lines connecting two minerals with different Rb/Sr ratios in slow cooling rocks. However, not all of these ages have geological significance since complete isotopic equilibrium is often not achieved in the cooling history. It may be asked how we can obtain the exact or approximate closure ages for minerals in the Rb-Sr dating system. Based on discussions from both graphical and mathematical models, some criteria are suggested: 1. The exact closure temperature age is obtained from the slope of the tie line between whole rock and the first mineral to close. Moreover, if the closure temperature of the first mineral to close is very near the emplacement temperature of rock, the closure temperature age of the first mineral to close should be close to the emplacement age of the plutonic rock. 2.The ages obtained connecting the minerals and the whole rock should be regarded as the best estimations for the closure temperature ages of these minerals. Moreover, the estimated ages shall be considered as the upper limit of the age of closure temperature for that mineral when (Rb/Sr) >(Rb/Sr) and the lower limit of the age for that mineral when (Rb/Sr) < (Rb/Sr). 3. Some errors in the closure temperature age may possibly occur if the age is from the tie line connecting the mineral and the first mineral to close, especially, when the mineral of late closure has a much higher Rb/Sr ratio than that of the first mineral to close.