Formation of monodisperse right trigonal-bipyramidal (rTriBP) and cube-shaped CdSe nanocrystals─both being encased with six (100) facets─is found to be dictated by type of stacking faults along the (111) direction of the zinc-blende structure and an ideal facet-ligand pairing for the (100) facets. During growth with little kinetic overdriving, seeds with single twin boundary (TB) and single intrinsic stacking fault (ISF) grow into rTriBP and cube-shaped nanocrystals, respectively, through two consecutive stages. During the facet-formation stage, each seed would grow rapidly into the smallest faceted one to contain the ∼3 nm seed, with cube-shaped ones growing much faster than rTriBP ones because of the stacking-fault-dependent seed location in the final faceted nanocrystals. In the following facet-growth stage, cube-shaped nanocrystals also grow faster, presumably due to the highly reactive stacking fault edges. Consistent with this hypothesis, growth of rTriBP nanocrystals can become faster than that of cube-shaped ones by intentionally introducing additional intrinsic stacking fault(s) in the seeds. Cube-shaped and rTriBP CdSe nanocrystals exhibit distinctive optical properties, representing two classes of optical materials.