Nanoparticles (NPs) and colloidal nanocrystal clusters (CNCs) of ZnFe2O4 were synthesized by using a solvothermal method in a controlled manner through simply adjusting the solvents. When a glycerol/water mixture was used as the solvent, ZnFe2O4 NPs were obtained. However, using ethylene glycol solvent yielded well-dispersed ZnFe2O4 CNCs. X-ray diffraction (XRD) and transmission electron microscopy (TEM) data confirmed that the ZnFe2O4 NPs were a single crystalline phase with tunable sizes ranging from 12 to 20 nm, while the ZnFe2O4 CNCs of submicrometer size consisted of single-crystalline nanosheets. Magnetic measurement results showed that the ZnFe2O4 NPs were ferromagnetic with a very small hysteresis loop at room temperature. However, CNCs displayed a superparamagnetic behavior due to preferred orientations of the nanosheets. Electrochemical sensing properties showed that both the size of the NPs and the structure of the CNCs had a great influence on their electrochemical properties in the reduction of H2O2. Based on the experimental results, the formation mechanisms of both the ZnFe2O4 CNCs and NPs as well as their structure-property relationship were discussed.