Abstract In this study, cobalt-doped ZnO nanoparticles were synthesized by combustion method. Mixtures of citric acid and glycine were used as fuel. As-prepared powders were characterized by XRD, XPS, SEM-EDX, TEM and spectrophotometer. XRD patterns indicated that combustion reaction by different fuel mixture resulted in the formation of pure ZnO phase. However, citric acid combustion alone led to amorphous powder. Scherrer's equation demonstrated that the crystallite size increases with citric acid/glycine (C/G) ratio (38–61 nm). SEM and TEM images illustrated that the morphology of the powder depends on the C/G ratios and changes from rod-like to spongy hexagonal particles. Reflectance spectra showed that by higher C/G ratios, deeper green colors are obtained.