The formation pathway of aqueous-phase colloidal semiconductor magic-size clusters (MSCs) remains unrevealed. In the present work, we demonstrate, for the first time, a precursor compound (PC)-enabled formation pathway of aqueous-phase CdSe MSCs exhibiting a sharp absorption peaking at about 420 nm (MSC-420). The CdSe MSC-420 is synthesized with CdCl₂ and selenourea as the respective Cd and Se sources, and with 3-mercaptopropionic acid or L-cysteine as a ligand. Absorption featureless CdSe PCs form first in the aqueous reaction batches, which transform to MSC-420 in the presence of primary amines. The coordination between primary amine and Cd²⁺ on PCs may be responsible to the PC-to-MSC transformation. Upon increasing the reactant concentrations or decreasing the CdCl₂-ligand feed molar ratios, the Cd precursor self-assembles into large aggregates, which may encapsulate the resulting CdSe PCs and inhibit their transformation to MSC-420. The present study sheds essential light on the syntheses and formation mechanisms of nanocrystals.