In this work, MgO-ZrO2 and MgO-6YSZ ceramic fibers were prepared with sol-gel method via electrospinning. Polymorph stability and microstructure evolution of zirconia fibers were fully characterized by X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, and Scanning electron microscope. The results indicated that tetragonal zirconia for MgO-ZrO2 was obtained and cubic zirconia could be fully stabilized for MgO-6YSZ with MgO molar fractions varying from 0.1 to 0.5 at 800 degrees C. Monoclinic phase appeared with MgO molar fractions even up to 0.5 for MgO-ZrO2 system and partially or fully stabilized zirconia could be achieved for MgO-6YSZ at 1000 degrees C and 1200 degrees C. Grain size was gradually decreased with increasing of MgO content at 800 degrees C both for MgO-ZrO2 and MgO-6YSZ ceramic fibers. The grain size of both systems increased with MgO molar fractions varying from 0.1 to 0.2 and then decreased at higher contents at 1000 degrees C and 1200 degrees C. A discussion on relationship among MgO state and the phase stability and grain size was presented. This work shows surface excess and solid solution of MgO predominantly controlled the phase stability and microstructure evolution of zirconia fibers.