Abstract: Poly(ethylene terephthalate)-Poly (tetramethylene ether) multiblock copolymer sample was fractionated by cross fractionation using two different solvent/nonsolvent systems. Eighteen fractions with different molecular weights and different chemical compositions were obtained. These fractions were employed to study the solution properties and characteristics of PET-PTMG copolymer. The composition distribution and molecular weight distribution were constructed from the cross fractionation data and SEC measurement. The comparison between the results of two methods have been made. The behaviours of cross fractionation and SEC of PET-PTMG have been discussed. The findings are as follows: 1. Cross fractionation trends to separation according to composition for the copo- lymer with highly incompatibility of components and can not deduce the true molecular weight distribution. 2. SEC determination gives only information on the composition drift as function of apparent molecular weight but not the true composition distribution. 3. The SEC universal calibration method is not applicable for PET-PTMG multi- block copolymer in CHCl_3.