A study on hexaaluminates La0.8A0.2NiAl11O19 (A = Ca, Mg, Sr) as catalysts for CO2 reforming of CH4 was carried out by means of the XRD, TPR, TGA, XPS, and TEM techniques. It was found that the active Ni dispersion and Ni particle size depend strongly on the ionic radius of the alkaline earth metal (Ca, Mg, Sr). The ionic radius of Ca is less than that of La, and the difference between the ionic radii of Ca and La is lowest. The La0.8Ca0.2NiAl11O19 catalyst exhibited the highest activity compared to those of the other two catalysts, and it also showed a remarkably low carbon deposition. The presence of two types of carbon (deactivating encapsulating carbon and nondeactivating filamentous carbon) was observed by TEM. The filamentous carbon formed more easily over the La0.8Ca0.2NiAl11O19 catalyst.