Self-activation is a process that takes advantage of the gases emitted from the pyrolysis process of biomass to activate the converted carbon, which saves the cost of activating agents and decreases the environmental impact, compared with conventional activation processes. An activation model was developed to describe the mechanism of the activation process, and it was examined by the self-activation experiments using the kenaf core as a raw material. The relationships among the parameters, yields, specific surface areas, and specific pore volumes were quantified. The results showed that the ideal temperatures for the self-activation process of the kenaf core were found between 970–1090 °C. The yield of 9.0% for the activated carbon from the kenaf core provided the highest surface area per gram of biomass, while the yields of 5.5–13.8% could achieve 90% of the highest. The developed activation model can be used to explain the relationship between the yields, specific surface areas, and specific pore volumes, effectively.