We have observed a single CdSe quantum dot (QD) by high-resolution plan-view transmission electron microscopy images to investigate the shape and size of CdSe QDs naturally formed on ZnSe(0 0 1) surfaces. A tentative model is that the base structure of a CdSe dot is rhombic with the long axis of about 20 nm in length along the [1 (1) over bar 0] direction. The lattice spacing inside the CdSe dot is completely the same as that outside the CdSe dot. The polarization dependence of photoluminescence (PL) from CdSe QDs is taken to investigate the effect of the QD shape on the optical property. The PL spectra are strongly polarized parallel to the [1 (1) over bar 0] direction, which is consistent with the proposed model of CdSe QDs. The CdSe coverage dependence of polarization PL spectra indicates that the PL polarization degree increases with QD formation.