Carbon spheres with different sizes has been synthesized through a hydrothermal method using glucose and water. Then single-factor contrast experiments about the size of carbon spheres, the concentration of KMnO₄ and the temperature of the reaction were carried out to probe into the effect of these factors on the morphology of the MnO₂ products. Kirkendall Effect is an effective theory to interprete the result, and though a series of replication experiments, we came to the final conclusion: The flower-like MnO₂ can be fabricated through the reaction of low-concentration KMnO₄ and small-size carbon spheres; The hollow-sphere-structure MnO₂ can be obtained by the reaction of low-concentration KMnO₄ and big-size carbon spheres; The porous solid-structure MnO₂ corresponds to high-concentration KMnO4, whatever size the carbon spheres are; Tempetature is not a notable fator.