Dendrite, which plays a key role in determining the mechanical properties of the casting parts, is the most important growth form in cast ingot and its pattern has received a wide range of investigations. In this work, the three-dimensional (3D) dendritic morphology of primary phase Cu₂Mg in Cu-10.25%Mg hypereutectic alloy has been studied using the serial sectioning technique during directional solidification, due to the fact that the Cu₂Mg phase is a typical Laves phase existing in most intermetallic compounds. The experimental results showed that the secondary dendrite profile of primary phase Cu₂Mg had different morphologies at the pulling rate of 20 μm/s, which included plate-like, octahedral and faceted patterns. The occurrence of these patterns was related to alloy macro--segregation and the thermo--solutal convection during directional solidification. The growth dendrites and their side-branching morphologies in the three-dimensional section were clearly interpreted differently from those in the two-dimensional section. The two-dimensionally discontinuous dendrites in growth direction were the secondary dendrites of primary phase Cu₂Mg and the branching dendrites in two-dimensional section were part of the primary dendrites. Furthermore, the primary and secondary dendrite arm spacing were calculated as 112.53 and 40.78 μm, close to those of the plate-like dendrites measured at the solidified fraction of 0.6 with the values of 123.30 and 44.82 μm, less than those of the octahedral dendrites measured at the solidified fraction of 0.8, which were 198.00 and 47.66 μm. In addition, the growth rates of secondary dendrites with octahedral, faceted and plate-like patterns were calculated as 1.40, 1.94 and 2.66 μm/s, respectively, an order of magnitude less than the pulling rate of 20 μm/s.