A ceramic (SiOC) and carbon nanotube (CNT) composite (SiOC/CNT) is fabricated by a one-step process via the in situ growth of CNTs in a ceramic body during its formation from a polymer precursor (polysiloxane) by pyrolysis. The in situ growth of CNTs is realized by premixing a metal catalyst in the precursor and adding a hydrocarbon (ethanol) at a certain pyrolysis temperature. The CNTs grow inside the pores that are developed in the ceramics at the intermediate stage of pyrolysis through catalytic cracking of the hydrocarbon by the metal catalyst particles nestling in the pores during ceramic formation. The subsequent pyrolysis at a higher temperature leads to densification of the ceramic dispersed with the in situ-grown CNTs. Microstructural observation reveals a homogeneous growth of crystallized multiple-walled CNTs in nanosize pores formed in the ceramic. The in situ-grown CNTs combine well with the ceramic matrix that forms a strong interface bonding. The in situ-grown CNTs increase the electrical conductivity of the ceramics by eight orders of magnitude compared with a CNT-free SiOC ceramic. This in situ growth strategy can be generalized as the fabrication of ceramic-CNT composites with promising mechanical and electrical properties.