The evolution of gamma/gamma' microstructure in elastically constrained Ni-8.5at.%Al-5.4at.%Ti alloy was investigated by means of transmission electron microscopy and the energy calculations based on a microelasticity theory. When the homogenized Ni-Al-Ti alloy is aged at 1213 K, cuboidal gamma' precipitate particles appear throughout the gamma matrix. When the gamma/gamma' two-phase microstructure is aged at 1023 K or 973 K, the phase separation takes place in each gamma' precipitate and a number of gamma particles newly appear in the cuboidal,gamma' precipitate. In the course of further ageing at 1023 K or 973 K, the gamma particles coarsen and become plates with surfaces parallel to {100}, then some gamma plates extend along one of {100} to become flatter, and finally reaches the gamma matrix surrounding the gamma' precipitate. Not only the elastic energies but also the chemical free energy plays an essential role in this type of split as the driving force. The split takes place throughout the microstructure although the volume fraction is high, which results in the occurrence of the decelerated coarsening of gamma/gamma' two-phase microstructure in an elastically constrained Ni-Al-Ti alloy.