Differential scanning calorimetry and polarized optical microscopy were used in this study to identify the contributions and interactions of both primary and secondary crystallization processes during the isothermal crystallization of a PEKK 70/30. Primary crystallization, which is related to the growth of spherulites, was monitored by polarized optical microscopy. The data collected allowed identifying the corresponding nucleation density and crystal growth rate that were subsequently used to feed a kinetic model derived from Hillier’s equation and already been reported in literature. The DSC data were then used to determine the contribution of the secondary crystallization mechanism, considering that primary crystallization is related to the instantaneous nucleation and growth of spherulites. From these data, an inverse approach was used to identify the few remaining parameters of the model. The proposed approach has the advantage of providing kinetic parameters representative of the secondary crystallization mechanism that are not dependent on the inverse identification procedure. Doing so, a non-integer Avrami exponent equal to 2.7 is obtained and discussed.