An increase of flexural modulus was observed by blending liquid organic phosphorous compounds, having specific structure, with polyphenylene-ether (PPE) or polycarbonate (PC). Blending 35 wt % of bisphenol-A type compound with two phosphate atoms in a molecule was especially significant. Flexural modulus increases of 12 and 32% were observed for PPE and PC, respectively. In the case of PPE, the increase tended to depend on the molecular weight of organic phosphorous compounds. The analysis of polymer chain motion of PPE by dynamic viscoelastic spectrum showed a change in shape at the P-dispersion temperature zone. From the trend of activation energy of the dispersion peak top and temperature dependence of both stock and loss modulus, it was concluded that the organic phosphorous compounds formed a complex bonding with the polymer chain with subsequent restricted P-type chain motion. As a result, the increase of modulus was observed.