Fluorescence resonance energy transfer(FRET)is increasingly used to study inter-and intramolecular interactions in living cells.Since being proportional to the concentration of the donor-acceptor complex.FRET value must be normalized to exclude the influence of the ratio and the concentration of donor and acceptor for comparison.Different from the intra.molecular FRET which is simplified by the fact that the COncentration of the donor is equal to that of the acceptor,the inter-molecular FRET is usually too complicated for most existing measurements to quantify exactly.We deduced the exact proportion of the donor-acceptor complex based on a unique characteristic of homodimer,a special kind of the intermolecular interaction,developed an exact quantification measurement of the FRET.We proved the novel method Can generate more reliable estimation of FRET value by comparison with other methods using a homodimer,estrogen receptor alpha(ERa),as a FRET pair.