Nanoparticles of ZnSe were obtained by colloidal synthesis by using Extran as surfactant. ZnSe free of by-products was successfully obtained after various cycles of the cleaning process with HCl. The x-ray diffraction results demonstrated that the NPs-ZnSe exhibit a cubic phase structure. The crystallite size of the obtained nanoparticles was estimated between 3 nm and 4.7 nm, these values are in agreement with those found by HRTEM. X-ray diffraction shows that the molar concentration influences the crystal size; smaller crystals are obtained when the molar concentration of Zn is larger than Se. Furthermore, SEM measurements show that the molar concentration affects surface morphology of the agglomerates. Variations of the pH value do not affect the NPs-ZnSe size. The presence of zinc and selenium was confirmed by the elemental analysis by EDS technique. Acknowledgements The authors wish to thank the VIEP-BUAP for their financial support and also to CNyN-UNAM for using JEOL JEM-2010 HRTEM.