A method for fraction fractionation of a vegetable protein such as soybean protein which comprises subjecting a source of the vegetable protein in an aqueous system to reduction conditions such as treatment with a sulfite compound, a glutathione compound or cysteine compound, or electrolytic reduction at pH within a neutral or alkaline range and then bringing the system to pH of 5.5 to 7.0 at a temperature of 20.degree. C. or lower to fractionate the system into a soluble or dispersing fraction and an insoluble or precipitate fraction.