Nanoparticles exhibit unique physical properties that are not found in their bulk counterpart. Cadmium selenid nanoparticles are synthesized with different sizes by the organometllic pyrolysis method after modification. X-ray scattering are used to determine the average size and the internal structure of the CdSe nanoparticles. The transmission electron microscopy (TEM) images of the samples show that the mean sizes of QDs are 4 nm. The effect of particle sizes on the optical properties is confirmed from UV–visible and fluorescence spectroscopic data. The CdSe nanoparticle exhibits red shift excitonic energy band at wavelength of ~500 nm to ~600 nm. The optical band gap decreases with increasing the particle size.