Two kind of polyimide/silica nanocomposites which were recorded poly(amic acid)/ silica(PAA/SiO2) and SiO2 in poly(amic acid) ammonium salt(PAS/SiO2) respectively, have been prepared for different methods. The size of SiO2 in PAA/SiO2 and PAS/SiO2 are measured by Particle size analysis and the average particle size are 377nm and 53.7nm. Transmission electron microscopy(TEM) images of PAS/ SiO2 confirm that the size of particles are ranged from 40 to 80nm. The stuctures of samples have been characterized by FT-IR spectra. The thermal stability of PAA/SiO2 and PAS/SiO2 are studied by thermogravimetric analysis (TGA) and the thermal behaviors were investigated by differential scanning calorimetry (DSC) in this paper. It is shown that PAA/SiO2 have slightly better thermal properties than that of PAS/SiO2. The results obtained from Instron universal strength tester present the water-solube PAS/SiO2 own the excellent mechanical prperties as same as PAA/SiO2.