Continuous ordering and spinodal decomposition of Ni rich Ni-Al system was studied theoretically by computer simulation and experimentally by transmission electron microscopy and 0.2% offset yield strength. Extending Gorsk-Bragg-Williams approximation to include the next nearest neighbor interactions in the $Ll_2$ type structure, ordering instability temperature, chemical and coherent spinodal lines were obtained. As the ratio $W_2/W_1$ of the next nearest to nearest neighbor interaction energy became more negative, the spinodal region moved to higher temperatures. Ordering instability temperature occurred above the spinodal region irrespective of the $W_2/W_1$ ratio. It is shown that the calculated phase diagram of Ni rich Ni-Al system is suited to the published results rather well when $W_1=10000$ and $W_2/W_1=-0.25$ are taken. From the results of computer simulation it is concluded that Ni rich Ni-Al alloys first lower their free energy by means of partial ordering and then undergoes a spinodal reaction involving further continuous ordering within the spinodal region. Outside the spinodal region, phase transformation by the nucleation and growth mechanism takes place. In addition to the theoretical studies, TE micrograph of Ni-16at.%Al alloy aged at 550$^\circ{C}$ revealed that homogeneous (continuous) transformation occurred. Superlattice spots were also observed in the TE diffraction pattern. The yield strength of Ni-14at.%Al alloy aged at 500$^\circ{C}$ increased about by two fold during very short aging and it then was followed by 2 step strengthening. It is reckoned that partial ordering is responsible for the first step strengthening and both spinodal decomposition and continuous ordering are responsible for the second one.
