Abstract: Fatty acid potassium is an important surfactant for daily chemical products and cosmetics industries with high foaming, soft texture and other advantages. In recent years, as a "green" soft matter, the fatty acid potassium gel has been applied in drug carriers, nanomaterials and phase change materials. However, many important aspects such as the crystalline structure, formation mechanism and thermal stability of fatty acid potassium gel remain unclear. This paper investigated the crystalline structure, rheological property and thermal stability of potassium laurate (LK) and potassium palmitate (PK) gels. At room temperature, polarized optical micrographs revealed strip-like crystals with cross stacking in 30%LK gel and linear crystalline fiber bundles in 30%PK gel. LK and PK gel formed at 30%LK concentration and 6%PK concentration, respectively. This was mainly ascribed to the stronger hydrophobic effect of the longer alkyl chain of PK than that of LK. X-ray diffraction showed that LK molecules of different concentrations retained the hydrated lamellar crystallites, while PK hydrated lamellar crystallites changed into anhydrous lamellar crystals with increasing PK concentration. Upon heating, LK hydrated lamellar crystallites transformed into hexagonal liquid crystallites and spherical micelles, and PK anhydrous lamellar crystallites changed into lamellar liquid crystallites. PK gel had basically stable storage and loss modulus at 20—60 ℃, which were 10 times higher than those of LK gel. These results indicated that PK gel formed by long alkyl chain fatty acid potassium had stronger crystalline network structure and higher thermal stability than those of LK gel.