The phase transition behavior of polystyrene／trans-decalin polymer solutions was observed under pressure with a hand-made high-pressure light scattering apparatus．The relationship between the phase transition temperature and the pressure was found

i．e．

the results show that the transition temperature increases with pressure．We also obtained phase diagrams in the T-φ plane and P-φ plane

but we could not get the critical pressure of the phase diagram in P-φ plane at low temperature under normal pressure．In these cases critical pressures can be obtained by extrapolation to the negative pressure range．