Positron annihilation lifetime spectra on polystyrene (PS) have been measured as a function of pressure relaxation time (t = 0-100 h) under the initial uniaxial pressure P = 0, 25 and 50 MPa, respectively. In order to extract the information on the variations in the microstructure and free-volume hole as a function of pressure relaxation time t, we first considered the inhibition of ortho-positromium (o-Ps) formation by positron-irradiation induced free-radicals; second we evaluated the quantity Delta I-3(t), i.e. the difference in the o-Ps formation probability between P = 25 (or 50) and 0 MPa. We observed that Delta I-3(t) dramatically increases with t up to a characteristic time (approximate to 5 and 12 h for P = 25 and 50 MPa, respectively) and then tends to a constant level. This result is explained by the evolution of microstructure and free-volume holes in PS during pressure relaxation.