We investigated the density dependence of the optical Kerr effect of two linear molecules, CO2 and N2O, by the optical Kerr effect experiment for a wide range of densities higher than the critical density. We also performed molecular dynamics simulation for CO2 in order to analyze the mechanism of the optical response. The density was controlled from the critical density rho(c)=0.467 g/cm(3) to 2.8 times rho(c) at a supercritical temperature of 323 K. We found good agreement between the experimental and simulation results. The relative importance of orientational dynamics of molecules and the collision-induced polarizability to the optical Kerr effect was studied at the molecular level. In particular, the collective orientational dynamics of the molecules as compared to that of the single molecule and the effect of the dipole-induced-dipole on the optical response function were analyzed in detail. Furthermore, the correspondence between the transition of the response function and the transition of the static structure as a function of density is discussed. (C) 2002 American Institute of Physics.