PURPOSE: To reduce the particle size of a high-molecular-weight high-viscosity cellulose ether without substantially lowering its viscosity by grinding it in a high-speed air stream impact mill.

CONSTITUTION: The production process comprises lowering the particle size of a high-molecular-weight high-viscosity cellulose ether without substantially changing its viscosity, and includes the step of grinding the cellulose ether in a substantially dry state in a high-speed air stream impact mill (the cellulose ether has a viscosity of about 100,000cP or above at 20°C before it is ground). The cellulose ether which is the most suitable is a methylcellulose having 27-32 wt.% methoxy substituents or a hydroxypropylcellulose having 4-32 wt.% hydroxypropyl substituents and 16-30 wt.% methoxy substituents. The cellulose ether product obtained by the grinding has a particle size of about 40-400 U.S. mesh.