To realize the precise modulation among the cesium lead halide perovskite-related phases is one of the most fascinating subjects and has motivated increasing research. The formation mechanisms of different phases of cesium lead halide have not been fully recognized. In this work, we reported the phase-selective synthesis of CsPbBr 3 nanorods and Cs 4 PbBr 6 nanocrystals (NCs) in amine-free systems, simply by adjusting the dosage of oleic acid (OA). By utilizing UV-visible absorption spectroscopy, we probed the evolution of the NCs and some lead bromide complex species during syntheses in reaction systems with different OA dosages. An OA-limited condition facilitated the formation of [PbBr 4 ] 2- and Cs 4 PbBr 6 NCs. OA-rich conditions facilitated the formation of [PbBr 3 ] - , [Pb 2 Br 5 ] - , etc. at early stages and the subsequent generation of CsPbBr 3 NCs. As the reaction systems equilibrated at a later stage, as-prepared CsPbBr 3 or Cs 4 PbBr 6 NCs would coexist with certain lead bromide complexes. OA dosage also greatly affected the kinetics of reactions toward CsPbBr 3 NCs. A relatively large amount of OA would accelerate the formation of CsPbBr 3 NCs. Our experimental results support two-step formation pathways of the cesium lead bromide NCs going through lead bromide complexes, and suggest that OA exerted all the influence by virtue of the lead bromide complexes. Our study presents a relatively clear picture of the formation of CsPbBr 3 and Cs 4 PbBr 6 NCs, which should be helpful in improving the preparation of lead halide perovskite-related NCs.