A viscous composition for transdermal drug delivery, a method of preparing the same, and a microstructure device prepared using the viscous composition are provided. Here, the viscous composition includes colloidal particles formed of a combination of a drug and a biodegradable polymer, and the biodegradable polymer includes a first amphiphilic polymer. The use of a solvent can be minimized to homogenize the drug in the biodegradable polymer including the first amphiphilic polymer, thereby forming colloidal particles. Therefore, since a separate solvent cannot be used, the microstructure device is expected to be a transdermal drug delivery system which is safe, efficient or useful for drugs which do not include a proper solvent, show poor bioavailability or have a high molecular weight.