Halloysite transforms to gamma-alumina or spinel (gamma-alumina-silica solid solution) by heating at 1100 to 1200oC and to mullite by subsequently heating at 1390oC for 10 to 50hrs. The crystallite size of formed mullite is nanometric. The purpose of this study is to analyze the formation process of mullite with nanocrystallite from natural clay (halloysite) using X-ray diffraction and transmission electron microscopy (TEM). At first Halloysite (from Democratic and People's Republic of Algeria.) was fired at 750oC for 1hr to dehydrate. Most of clay minerals have hydrate water in the structure and the water easily dehydrates by firing. This dehydrating process often makes the clay minerals amorphous. The amorphous powder dehydrated from halloysite by firing was used as a raw material to investigate the formation process of mullite with its nanocrystallite by the thermal treatment. The lower treatment below 1400oC was performed to prevent the crystallite growth. The heating at 1100 to 1200oC by conventional electric furnace makes gamma-alumina inside the amorphous form and the subsequent heating at 1390oC for 10hrs gives mullite with 20nm in diameter and 100nm in length of the crystallite with amorphous silica observed by TEM. The thermal treatment below 1400oC makes a possible to keep the mullite with nanocrystallite. To fabricate the dense form of mullite with nanocrystallite from the amorphous powder, spark plasma sintering (SPS) as one of rapid consolidating process was applied below at 1400oC for 10minutes under applied pressure of 40MPa to prevent any significant crystallite growth. The consolidation at 900 to 1000o C gave the fully dense form of amorphous silica and mullite with the crystallite size of 45nm. The thermal treatment at 800oC by SPS makes gamma-alumina inside the amorphous powder. The gamma-alumina (or spinel) and mullite crystallites with nano-size in the sintered specimens were characterized by X-ray diffraction and TEM.