The problem of using aluminium alloys with rare-earth metals (REM) obtained by conventional casting methods (cooling rate $\sim$(1–10) K/s) is a formation of large-size primary intermetallides ($\sim$100 $\mu$m) of various shapes. Now, production of high-quality aluminium alloys doped by rare-earth metals is based on grinding these inclusions. Corresponding techniques are expensive. Constant magnetic field with flux density 0.1 T is used to act on the shapes and sizes of primary intermetallides in a cast alloy based on Al–Cu system with 13% mass. REM. Such action is easy to implement and it is inexpensive. Alloy samples are obtained in cooling rates 4, 10, 11, 15 and 30 K/s. Magnetic field is applied to the alloy during its cooling and solidification. The metallographic method is used to study the cast structure of samples, obtained without and under influence of a magnetic field. The alloy contains intermetallides with different sizes and morphology. On the surface sections, inclusions with characteristic forms are observed, they are combined into three groups: 1—faceted, 2—with wave contours, 3—contained cracks, integrity breakdowns and inclusions fractured on fragments. As determined, an increase of cooling rate results rising in the number of inclusions of group 3. The magnetic field intensifies this tendency, increases the cracks number in crystals and reduces the size of the formed fragments. As a result, the grinding of majority intermetallides occurs due to their destruction into compact fragments with sizes of 3–15 $\mu$m. The magnetic field also contributes to dispersion of inclusions parts in the alloy volume.