In order to investigate the influencing factors in flotation fractionation, flotations were performed at varied conditions. The selectivity of fines fractionation was mainly affected by long fiber flocculation degree and if there were not sufficient flocculation of long fibers, more loss of long fibers could not be avoided. The amount of flotation rejects were totally dependent on the stability of froth floated on the stock surface. Only small size fines could stabilize the froth as they hindered the drainage of liquid lamella in flotation-froth. More flotation reject and better flocculation of long fibers were two important factors for improving flotation. Changing a flotation flux or an air-mixing ratio to increase the flocculation of fibers increased long fiber ratio in the reject. In order to satisfy the both conditions of reducing long fiber loss and of increasing flotation reject, search of fractionation promoter is needed.