The parent compound LaMnO3 of the hole-doped colossal magnetoresistance materials La1-xAxMnO3 (A = Ca, Sr, Ba etc.) has attracted much interest from condensed matter scientists. Stoichiometric LaMnO3 in which Mn ions are formally Mn3+, has an antiferromagnetic insulating ground state. Cooperative Hahn-Teller distortion removes the degeneracy of of the eg orbitals in the t2g3eg1 electron configuration of the Mn3+ ions, and stabilizes d3x2-r2/d3y2-r2 orbitals which are ordered. The orbital order consists of ordering of d3x2-r2 and d3y2-r2 orbitals in an alternate staggered pattern in the a-b plane. The pattern repeats itself along the c axis. We propose high-pressure investigation of LaMnO3 at high temperature. We propose to determine the pressure-temperature phase diagram of the Jahn-Teller transition in LaMnO3. We request 5 days of neutron beam time on PEARL for the proposed experiment.