Rambutan-like heterostructures consisting of Ni microspheres coated with oriented multiwall carbon nanotubes (MWCNTs) were synthesized by the one-pot thermal decomposition of a mixture of organic matter and Ni precursors. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy were used to reveal the formation mechanism. The growth of MWCNTs capped by Ni nanoparticles on the surface of the Ni nanoparticle-built microspheres followed a tip-growth mode. The composition and morphology of the rambutan-like heterostructures were easily controlled by changing the reaction time, mass ratio δ of polyethylene glycol (PEG) 20 000 to NiO, as well as type of C source and Ni precursor. Increasing the δ favored not only the increased C mass fraction but also the morphological conversion from Ni/C film core–shell structures to rambutan-like Ni/MWCNT heterostructures. Such changes caused the decreased saturation magnetization and enhanced permittivity properties with δ. Owing to intensive eddy current loss and multiresonance behaviors, rambutan-like Ni/MWCNT heterostructures with long MWCNTs exhibited significantly improved complex permeability and magnetic loss. Ni/MWCNT heterostructures coated by short MWCNTs showed an optimal microwave absorption property with a minimum R_L value of −37.9 dB occurring at 12.8 GHz. This work provides effective guidelines for devising and synthesizing highly efficient microwave-absorbing materials.