The direct mix of aqueous ferric chloride and o-phenylenediamine (OPD) solutions at room temperature has been demonstrated for the first time to be an effective, economic, and fast method for preparing microfibrils on a large scale. The formation of such large microfibrils is attributed to the self-assembly of the OPD dimers generated by the oxidation of OPD monomers by ferric chloride. It is also interesting that the resulting microfibrils can be broken into shorter ones by a simple sonication process and the final length of the microfibrils obtained can be controlled by varying the sonication time. The influences of both the amount of ferric chloride and the oxidant type on the size and the morphology of the microstructures are also examined.