Three different semi-IPN polyimide systems were prepared through blending in solution by using two different polyimides, such as poly (p-phenylenebiphenyl tetracarbonyimide) (BPDA-PDA) and poly (4,4＇-oxydiphylene pyromellitimide) [PMDA-ODA(E)I, and two different oligomers, such as bismaleimide (MDAB) and phenylthynyl-terminated BPDA-PDA (BPDA-PDA-PEPA) oligomers. These crosslinkable oligomers are used to modify the morphology of polyimides and, therefore, change the anisotropic properties of the resulting semi-TPN polyimide systems. The results show that the crosslinkable oligomers can disrupt the in-plane molecular arrangement which gives rise to an increase in lateral CTE and vertical CTE and a decrease in birefringence and the Young＇s modulus. The dielectric constant and water absorption increase with increasing MDAB content but decrease with increasing PEPA content. An empirical correlation between density and volume expansion is observed, indicating a correlation between the volume expansion and free volume expansion and free volume in the semi-IPN polyimide systems. The in-plane modulus decreases but the vertical moduli increases with increasing content of crosslinkable oligomers. In addition, the vertical modulus is affected by the free volume at low stress levels and can be enhanced by crosslinks or by a crystallinelike phase at higher stress :levels.