We have conducted an extensive study to investigate the effects of thermal treatments and dendrimers' structures on the chemical and physical properties of the surface-modified polyimide films. The amidation and cross-linking reaction between GO PAMAM dendrimers and polyimide were examined by XPS (X-ray photoelectron spectrometer) and FTIR-ATR (attenuated total reflection) measurements after thermal treatments under different temperatures. Moderate thermal treatment (120 degrees C) is proven to be able to induce the highly amidation reaction and to increase the degree of cross-linking on the polyimide surface. The gas separation performance of modified polyimide films is significantly improved due to the enhanced "molecular sieving" ability by dendrimer modification and the stronger interactions between the polyimide chains, such as covalent cross-linking bonding and hydrogen bonding. When the temperature of treatment reaches 250 degrees C, H-1 NMR and GPC test implied that the cross-linking structure between polyimide chains is broken and that the degradation of polyimide backbone chains also occurs. Gas permeation tests also indicated that high-temperature treatment of dendrimer-modified polyimide films is not beneficial to separation. In addition, the performance comparison between different dendrimers, PAMAM- and DAB-modified films, is carried out. The chemical structural differences in PAMAM and DAB dendrimers have also been verified as one of the important factors in determining the properties of modified polyimide films. At room temperature, PAMAM dendrimers show stronger cross-linking ability.