BNNTs grown in situ on the surface of silicon carbide fibers via a simplified ball milling, impregnation and annealing method using boron powder as the raw material were synthesized. The morphology, microstructure and chemical composition of as-grown BNNTs were well characterized by field-emission scanning electron microscopy, field emission transmission electron microscopy, electron energy loss spectroscopy and Raman spectroscopy. The analysis results indicate that the nanostructures are BNNTs with the diameter of 50–130 nm, exhibiting a multi-walled and bamboo-like structure and possessing two distinct morphologies of the tube walls: flat walls and bubble-chain walls, respectively. Based on the experimental characterization of BNNTs and as-milled powders, the possible growth process of BNNTs has been described and the formation mechanisms of two types of bamboo-like BNNTs have been proposed, including the tip growth mode and the base growth mode.