A method for producing a fiber assembly, including: putting a plurality of fibers and a fiber treatment agent into a stirring tank; and stirring a mixture of the fibers and the fiber treatment agent with a stirring blade to granulate the mixture, wherein the fibers include carbon fibers, and the mixture is granulated such that the fibers are aligned. A method for producing a fiber assembly, wherein a plurality of fibers containg carbon fibers and a liquid are put into a stirring tank and the mixture of the fibers and the liquid is stirred with a stirring blade, thereby obtaining a fiber assembly which has a spheroidal shape or a strand shape.