Abstract Isothermal crystallization kinetics of poly(ether ketone ketone) (PEKK) and its carbon-fibre-reinforced composites was analysed by using a phenomenological two-stage crystallization kinetics model modified from the Avrami equation. We have determined a different Avrami exponent for each crystallization stage from differential scanning calorimetry isothermal measurements. In the primary crystallization stage, the nucleation growth is assumed to be three-dimensional, sporadic and with an Avrami exponent of 4; whereas in the secondary stage, it is assumed to be two-dimensional or diffusion-controlled, with an Avrami exponent of 2. This model gives a considerably better fit than the unmodified Avrami equation and the two-stage Velisaris-Seferis model. The estimated final volume fraction crystallinity of the primary crystallization is approximately 60% over the temperature range of 298–314°C. Cold crystallization produces a higher value of the primary volume crystallinity ranging from 70% to 100% over the temperature range of 221-200°C. The presence of fibre in the composite does not change the relative primary volume crystallinity nor does it affect rate constants of both stages. The effect of fibre on the total crystallization rate of PEKK in the composite is also small.