Based on the theory of thermal conductivity, in this paper we derived a formula to estimate the prolongation period (ΔtL) of cooling-crystallization process of a granitic melt caused by latent heat of crystallization as follows: where TM is initial temperature of the granite melt, TC crystallization temperature of the granite melt, CP specific heat, Δtcol cooling period of a granite melt from its initial temperature (TM) to its crystallization temperature (TC), QL latent heat of the granite melt. The cooling period of the melt for the Fanshan granodiorite from its initial temperature (900°C) to crystallization temperature (600°C) could be estimated ~210,000 years if latent heat was not considered. Calculation for the Fanshan melt using the above formula yields a ΔtL value of ~190,000 years, which implies that the actual cooling period within the temperature range of 900°–600°C should be 400,000 years. This demonstrates that the latent heat produced from crystallization of the granitic melt is a key factor influencing the cooling-crystallization process of a granitic melt, prolongating the period of crystallization and resulting in the large emplacement-crystallization time difference (ECTD) in granite batholith.