Nanosized CdSe particles were synthesized at a temperature of 115°–175°C by a solution method with air pressure condition. Ethylene glycol (EG) was used as the main solvent and sodium selenite and cadmium nitrate‐tetrahydrate as inorganic sources. The influence of refluxing temperature and time on growth morphology and crystallization was investigated by transmission electron microscope, high‐resolution transmission electron microscope, and X‐ray diffraction. The chemical reaction was deducted based on X‐ray photoelectron spectra. The optical absorption property was measured by UV‐vis. The CdSe nanoparticles synthesized through this EG solvent system was single wurtzite crystallization and had a nanoscale size below 15 nm diameter with a narrow size distribution. The reduction of Se4+→Se0→Se2−and the disproportionation of Se0 occurred during the synthetic process and dominated the chemical reaction.