Viscoelastic properties of SAN-based composites. i.e., SAN/PPTA, SAN/KF and SAN/CF have been determined from a Rheovibron and RDS Rheometer as well. Reinforcing effects of PPTA and KF were much greater compared to those of CF, and this was interpreted in terms of strong interaction between SAN and PPTA, and SAN and KF. Strong interaction via hydrogen bonding was also confirmed from FTIR measurements. A significant difference was also true between SAN/PPTA, a molecular composite prepared from solution blending, and SAN/KF, a fiber reinforced composite, with much greater effect for the former at low filler concentration. This is presumably due to the microfibril formation of PPTA in SAN matrix leading to a fine dispersion with more hydrogen bonding chance, attested from FTIR measurement.