CdSe nanospheres and dendrites have been successfully synthesized via an ionic liquid-assisted hydrothermal synthetic method. The samples are characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Our experimental results demonstrate that the CdSe dendrites are obtained by self-assembly from nanospheres under the driving force of oriented attachment process. Moreover, we found that the sizes and morphologies of the crystals depended on the concentration of the ionic liquid, and we obtained the CdSe nanospheres with low concentration of [BMIm][SeO(2)(OCH(3))] and CdSe dendrites with high concentration of [BMIm][SeO(2)(OCH(3))]. The effects of [BMIm][SeO(2)(OCH(3))] on the formation of the CdSe have been investigated systematically. This method is expected to be a useful technique to prepare other metal selenides for controlling the diverse shapes and sizes.