The structure and compositions of spinodal phases in Alnico8 alloy isothermally treated at 830 °C under external magnetic field (H_ext) up to 10 T were investigated by transmission electron microscopy (TEM). Under ultra-high magnetic field (10 T), the spinodal structure of Alnico8 alloy presents interesting diversity for the shapes of α₁ phase particles, namely, Π-shape, H-shape and Ш-shape apart from regular rod-shape. Composition analysis shows that the Fe was mainly distributed in the α₁ phase. The Co content in the α₁ phase was lower than that in the α₂ phase as evidenced by the point analysis results, and Co content inα₁ phase of the alloy treated without magnetic field was higher than that of the alloy treated with a magnetic field. In contrast, the Ni, Al, Cu and Ti were concentrated in the α₂ phase. The average hyperfine field of the alloy treated without magnetic field was about 301.3 kOe. While the alloy treated under magnetic field, it was slightly decreased, which was about 292.0 and 296.1 kOe under 0.7 and 10 T, respectively. Magnetic properties of the alloy treated under a 10 T magnetic field were the best, which were about 1.09T, 396Oe and 9.5 kJ/m³ for B_r, H_c and (BH)_max respectively. Especially for the coercivity, which was mainly contributed by the anisotropy of the α₁ phase induced by the high magnetic field.