The thermodynamically stable long-range orbital order in bulk LaMnO3 becomes metastable at nanoscale around a critical particle size dC ̃20 nm. The orbital order–disorder transition switches from reversible to irreversible at dC while the resistance in the orbital ordered state decays by 2%–4% over a time scale of ∼3000s. At well below dC, of course, a stable orbital disordered phase emerges. The orthorhombic distortion of the underlying crystallographic structure (space group Pbnm) decreases systematically with the decrease in particle size and at far below dC (e.g., at ∼10 nm), the structure becomes cubic (space group Pm3−m). Using the crystallographic and electrical resistance data, a phase diagram has been constructed showing the evolution of different orbital phases as a function of particle size across ̃10 nm to bulk for stoichiometric LaMnO3.