Radial polymellitimides were prepared in two steps from mellitic trianhydride and diamines. In the first step of forming a soluble polyamic acid, homogeneous solution polymerization was carried out in a high polar solvent, such as dimethylformamide or dimethyl acetamide, as well as in a nonpolar solvent of tetrahydrofuran. By the tetrahydrofuran process, it was possible to give simultaneous formation and precipitation of polyamic acid free from the usual tightly bound solvents. The insoluble, thermally stable polymellitimides were formed from the polyamic acids by imidation reaction under heat and vacuum. The comparison of thermal and oxidative stability of polymellitimides was also made by DTA and TGA in air and nitrogen atmosphere. The results of these thermal analyses revealed that polymellitic imide ring was highly stable to thermal and oxidative decomposition, while the thermal stability was greatly affected by the structure of diamine components. Polyimides derived from aromatic diamines were proved to have higher thermal decomposition temperature than that of polyimide from aliphatic diamine. Polyimides with methylene or hexamethylene group in the main chain showed decreased thermal oxidative stability in air, while fully aromatic polyimides or polyimide with ether linkage between aromatic rings showed excellent resistance to thermal and oxidative decomposition.