Mullite-nickel nanocomposite fibers with Ni nanoparticles of controllable size, dispersion, and consequent magnetic properties were fabricated using sol-gel/electrospinning method, followed by thermal reduction. The fibers were electrospun from an aqueous solution containing sol-gel mullite precursor and nickel nitrate. These fibers were then heat treated in the reducing atmosphere between 550 degrees C and 750 degrees C to achieve fine-dis persed metallic Ni nanoparticles (NPs). After the Ni2+ was reduced to Ni NPs at 750 degrees C for 10 h, the fibers were then directly transformed to the mullite fibers at 1000 degrees C without the undesirable intermediate spinel phase. In many high-temperature applications, mullite is the desired phase than spinel. If not fully reduced, the Ni2+ cations induce early precipitation of spinel phase before mullite can be formed. This spinel phase was a solid solution between Al2NiO4 and Al-Si spinels, which later reacted with the residual silica and formed a mixture of mullite and spinel at 1400 degrees C. The formation of spinel phase was suppressed or fully eliminated with chemically reducing Ni2+ to metal NPs. The average size of nickel NPs within the fibers was similar to 20 nm, insensitive of the Ni concentration and reducing temperature. However, the Ni NPs on the fiber surface grew as large as similar to 80 nm due to fast surface diffusion. The magnetic nanocomposites exhibited ferromagnetism with saturation magnetization (M-s) close to pure nickel of the same nominal weight, but coercivity (H-c) much smaller than the bulk nickel, indicating the nature of bimodal magnetic nanoparticle distributions. The majority of small Ni NPs (similar to 20 nm) within the fibers exhibited superparamagnetism, while the minor portion of relatively large NPs (50-80 nm) showed ferromagnetism.