Alnico, an alloy of primarily Al, Ni, Co, and Fe, is an attractive, near-term, non-rare-earth permanent magnetic alloy for wind power generators and electric vehicle motors, due to its hightemperature stability and relatively low cost [1]. Theoretical work suggests that size refinement and morphology control of the magnetic phase could double the energy product of alnico [1]. The magnetic properties of alnico are closely related to the spinodal decomposition (SD) of the FeCo-rich (α1 phase) hard magnetic phase and a non-magnetic NiAl-rich phase (α2 phase) that form after a lengthy heat-treatment process: solutionization, magnetic-field annealing (MA) to induce anisotropic growth of the SD phases, and drawing to optimize the magnetic properties. The MA is critical to obtain optimal magnetic properties [1]. In this study, we performed a careful microstructure and magnetic property study on an isotropic (alnico 8H) 32.4Fe-38.1Co12.9Ni-7.3Al-6.4Ti-3.0Cu alloy at different stages during heat treatment. A combination of atom probe tomography (APT), orientation imaging microscopy (OIM), and transmission electron microscopy (TEM) techniques were used to elucidate the structural evolution of the SD phases in the alnico alloy and their effect on magnetic properties.