The Gusandong Tuff is an extensive rhyolitic ignimbrite that has been used as an excellent key bed in the Cretaceous Gyeongsang Basin, SE Korea. Magnetic fabric analysis in the tuff, using AMS (Anisotropy of Magnetic Susceptibility) technique, shows an anomalous fabric which has horizontal k3 and vertical k1 axes in several sites. We attempted a stepwise thermal treatment to investigate the changes of AMS axes and parameters (T, PJ) in four sites (KT11B, KT11M, KT18B, KT18M). The changing patterns of magnetic fabric during heating are classified into 4 main types: (1) Type-0 is characterized by unchanged three principal axes (k1, k2, k3) overall heating steps. (2) Type-I shows the exchange of k1 and k2 axes each other. Type-I can be classified as subtype Type-IS (axes are suddenly changed) and Type-IG (axes are gradually changed). (3) Type-II is characterized by the exchange of k2 and k3 axes each other. (4) Type-III shows that three principal axes switch their positions with one another. Type-III can be classified as subtype Type-IIIA (k1→k2, k2→k3, k3→k1) and Type-IIIB (k1→k3, k3→k2, k2→k3). The directional changes of the axes begin between 500-580°C and then terminate near 670°C. Except for Type-0 and Type-IS, most of the magnetic fabrics are transformed gradually. The changes of axis orientation and magnitude observed in this study can be reasonably explained with the theoretical inverse fabric model proposed by Rochette et al. (1992) and Ferre (2002). The magnetic fabrics of before heating, therefore, are inverse fabrics and the inverse fabric can be transformed gradually into the normal fabrics through thermal treatment. Other analysis (rock fabric analysis, rock magnetism and BSE images) are support this interpretation too. Before heating, rock magnetism shows that the main carrier of the AMS fabric is paramagnetic minerals with small amount of ferromagnetic mineral (except Type-0). The fabric is interpreted to be an inverse one attributed to single-domain (SD) or superparamagnetic (SP) magnetites based on day diagram analysis. Heating helps growing new pseudo-single domain (PSD) or multi-domain (MD) magnetites that are mimetic with respect to the biotite cleavage. This result indicates that the thermal treatment can be a strongly useful tool for eliminating the inverse AMS magnetic fabric of natural rocks.