A series of composite films composed of μm-sized aluminum nitride (AlN) particles and polyimides (PIs) were fabricated using (a) rigid-chain sBPDA-PPD (sBPPD) PI and (b) flexible-chain sBPDA-ODA (sBPOD) PI as matrices. The relationship between their morphological structures and thermophysical properties were investigated by analyzing cross-sectional SEM images and by measuring out-of-plane thermal conductivity (TC_⊥) and in-plane coefficients of thermal expansion (CTE_//). The sBPPD chains exhibited a better polymer-filler packing behavior due to their rigid structure having small free volume. Both TC_⊥ and CTE_// of AlN-filled sBPPD films showed positive deviations from theoretical predictions, whereas no such behavior was observed in AlN/sBPOD composites or in sBPPD films filled with highly anisotropic hexagonal boron nitride (hBN) flakes. The enhanced TC_⊥ and CTE_// observed for the sBPPD composites can be attributed to a significant change in the orientational state of the PI chains; in-plane orientated sBPPD chains in the pristine PI film become distorted by incorporation of isotropic AlN particles.