Polyamic acid precursors of high performance BTDA/MDA and PMDA/ODA were synthesized, and their thermal imidizations were investigated isothermally and non-isothermally by thermogravimetry. All the polyamic acids revealed a two-step imidization behavior, fast imidization at early stage and slow imidization at later stage. The onset temperature of imidization and Tg of the polyamic acids were determined using TGA, and thermal imidization parameters were investigated. Tg of the BTDA/MDA based polyamic acid was 105.8 $^{circ}C$, and that of the PMDA/ODA based polyamic acid was 122.6 $^{circ}C$. Intrinsic viscosity depression of polyamic acids with storage time was measured, and its dependence on the polymerization solvent was also investigated.