Aromatic polyimides have found a wide range of applications in advanced aerospace, automobile, microelectronic and printed circuit industries because of their distinguished thermal stability, high chemical resistance, good mechanical strength and excellent dielectric properties. However, conventional polyimides are often categorized as an insoluble, intractable, and infusible material owing to their rigid backbone structure. These disadvantages make them extremely difficult to process and greatly limit their commercial uses. Various attempts have been developed to overcome these deficiencies, including the use of noncoplanar or alicyclic monomers and the introduction of flexible segments into the polymer backbone. The strategies of these methods are the reduction of chain crystallinity, inter-molecular charge-transfer and electronic polarization interactions. An alternate successful approach involves the incorporation of pendant groups onto the rigid polyimide backbone. These works have shown that the presence of bulky groups could effectively prevent the coplanarity of aromatic rings and reduce the packing efficiency of molecular chains without sacrificing thermal properties. In this study, we synthesized two series of polyimides containing alkyl side groups in different sizes and investigated the effect of the molecular structure of pendant groups on the thermal and dielectric properties of polyimides.