Solvents are widely used in crystallization, but their effects on the shape development of crystals are under debate. Here, we report a view on how solvents play their role by considering the viscosity of solvents. We synthesize silver particles in a mixture of alcohol and water. The viscosity of the solvent is changed by varying the volume ratio of alcohol in water. With the variation of viscosity, diverse morphologies of silver particles are synthesized. Small cubic crystals are formed at low viscosity, while hierarchical flower-like particles are formed at high viscosity. Two alcohols are employed, namely ethanol and propanol. No matter which alcohol is employed, the results are similar. Due to the limitation of mass transfer at high viscosity, the particles synthesized in such solvent are smaller and stabilized by the solvent. When the solution containing these particles are dropped onto metal substrates, these tiny particles firstly aggregate, which is followed by classic crystallization, forming flower-like hierarchical structures. These findings show the importance of the viscosity of solvents in shaping particles, which is underestimated previously.