The specific refractive index increments (dn/dc) of short chain polymer exhibit pronounced molecular weight dependency due to the fact that end group contributes much to the optical property of whole polymer chain．From the weight fraction additivity rule．(dn/dc) of the whole polymer chain is the sum of those of corresponding structural units and it is indicated that changing the end group of a given polymer，the molecular weight and(dn/dc) of the polymer chain stem cou-ld be determined simultaneously by measuring (dn/dc) of the whole polymers．It is also possible to estimate the composition of the copolymer from (dn/dc) of the cha-in stem．This principle especially applicable to short chain polymers with pronoun-ted end group effect．This principle is checked by quantitative size exclusion chro-matography to measure (dn/dc) of the benzene solution of oxyethylene-oxypropylene copolymer with different end groups and satisfactory results are obtained．A method for simultaneously estimation of the molecular weight and composit-ion of short chain copolymer by measuring (dn/dc) of the sample solution with quantitative SEC was proposed．