It is well known that the variation in the crystallized fraction, x(t), can be determined directly by optical microscopy (OM) or indirectly by differential scanning calorimetry (DSC) under isothermal conditions. In this work, the kinetic crystallization parameters E (activation energy) and n (Avrami index) of a quasi-stoichiometric lithium disilicate glass (Li2O·2SiO2–LS2) were studied. First, bulk samples were heat treated in a vertical tube furnace between 500°C and 530°C for different periods of time, and were carefully prepared. Furthermore, the same samples were then heated in a calorimeter from room temperature to 850°C at 10°C/min. The following E and n values were obtained: (i) OM: 462.2 ± 1.23 kJ/mol and 4; and (ii) DSC: 187.5 ± 0.45 kJ/mol and 2.4. The results indicated that the activation energy was reduced due to the formation of new crystals and growing of the “preexisting crystals” during the DSC experiments, and that the predominant crystallization mechanism in the samples changed according to the chosen thermal history.