The effect of temperature on the Raman spectrum of MgSiO3-perovskite is investigated between 77 and 300 K. The data, combined with those obtained at high pressure, are used to infer microscopic anharmonic parameters (a(i)) It is shown that only two low-frequency Raman modes are significantly anharmonic. The small volume dependency of the a(i) parameters allows a quasi-harmonic calculation of the thermal pressure from a simplified density of states of the vibrational modes without other a priori informations. The calculated high-temperature EOS reproduces to within +/- 0.4 % the experimental molar volume obtained in the 0.1 MPa-36 GPa and 77-2000 K range. A softening of a low-frequency mode with decreasing temperature is also observed.