An in-depth understanding of the growth mechanism of nanocrystals (NCs) is of great significance for shape control of colloidal semiconductor nanostructures. In this study, we elucidate the formation mechanism of anisotropic CdSe NCs by systematically investigating their growth in the presence of dioctylamine and carboxylate ligands with different chain lengths. A morphological transition from nanotubes and nanosheets to irregular nanorods and finally to nanodots was observed when the carbon number of the carboxylate ligands increased from 2 to 18. The traditional understanding of the growth of anisotropic CdSe nanostructures is mainly based on the monomer addition, which is difficult to explain the phenomena observed. We found that both short- and long-chain carboxylate ligands can lead to the anisotropic growth of CdSe NCs by attachment of early-formed building blocks, which is a nonclassical particle-mediated growth approach. The use of different carboxylate ligands plays a key role in the formation of different building blocks by affecting the local monomer supersaturation. We provide both experimental observations and first-principles simulations to support this hypothesis.