Abstract: A formula for calculating M in a given interval of Wesslau distribution and Lansing-Kraemer distribution has been derived. It is impossible that the molecular weight of any polymer will distribute over the whole range of 0-∞, hencc a GPC curve always has two boundaries. Therefore a GPC curve should be treated as a normal distribution curve wih its side tails cut out. This paper has shown that three parame-ters (M₀, β and U_R ) are necessary for characterizing a normal GPC curve.The precision of M calculated from GPC curve by Wesslau function is discussed.