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Highly purified and uniformly distributed carbon nanotube (CNT) network is one of the key issues for developing advanced nanocomposites. However, the challenge for mitigation of the aggregation of CNT bundles during the CNT film fabrication still remained. In this work, debundling and reorganization of CNT bundles have been realized simultaneously by using a high-temperature thermal treatment. The debundling of large CNT bundles into small ones started after they were heat treated under 1400 °C and uniformly distributed CNT networks with small bundles were obtained after heat treated under 1800 °C. The tensile strength of the CNT film significantly increased by about 64% compared to the pristine film. Microstructural observations showed that the iron nanoparticles started to evaporate at 1400 °C and were completely removed at 1800 °C. Hollow amorphous carbon shells wrapped on the nanoparticles were left inside the CNT networks after the removal of the iron nanoparticles. Interestingly, these amorphous carbon shells act as carbon source for the deposition of a carbon layer on the CNT surface when the heat treatment temperature was up to 2000 °C, and they were completely consumed after being treated at 2800 °C. The microstructural evolution process of CNT bundles during heat treatment suggests the simple high-temperature treatment strategy could be applied for fabrication of highly purified CNT networks with well-distributed small bundles, enabling the development of advanced multifunctional nanocomposites in the near future. A high-temperature heat treatment is used to investigate the microstructure evolution and mechanical properties of CNT films. Benefiting from the high-purity and uniformly distributed CNT networks with small bundles after heat treated under 1800 °C, the tensile strength of the CNT film has significantly increased by ∼64% compared to the pristine film. When heat treated under 2800 °C, the hollow amorphous carbon shells are completely consumed and act as carbon source for deposition of carbon layer on the CNT surface. [Display omitted] • Debundling of large CNT bundles was observed after a high temperature treatment. • A regenerative carbon layer was deposited on CNT surface under high temperature. • The microstructural evolution of CNT networks under high temperature was proposed. [ABSTRACT FROM AUTHOR]