Changes in the morphology of γ′ precipitates in a γ/γ′ two-phase microstructure in Co-based superalloys, which occur through higher-order alloying, have been investigated. The shape of γ′ precipitates changes from a cuboid to an irregularly rounded shape when the Ta content increases to 4 at.%. The irregular shape of the γ′ precipitates changes to cuboid again upon further alloying with 2 at.% Mo. Changes in the morphology of γ′ precipitates are closely related to variations in the lattice misfit between γ and γ′ phases upon alloying. A lattice misfit increases with an increase in Ta content, which makes γ/γ′ interfaces incoherent when the lattice misfit exceeds a critical value. On the contrary, the lattice misfit decreases upon alloying with Mo. Changes in the lattice misfit upon alloying are well explained through the partition behavior of alloying elements and their atomic volume.