PURPOSE: To obtain fractions such having a sharp MW distribution and a sharp composition distribution and so suitably usable in quality control or the like, by cross-fractionating the titled copolymer with a p-xylene/butyl cellsolbe solvent in a specified manner.

CONSTITUTION: An ethylene/α-olefin copolymer to be tested is used in a state in which it is supported on a carrier (e.g., diatomaceous earth). In step i, the above copolymer is placed in a column and then fractionally extracted with a xylene/ cellusolve solvent having the composition ratio varied continuously within the range of 0/100W100/0 (volume ratio), while the extraction temperature is maintained constant at a temperature of 110°CWb.p. of xylene (fractionation on the basis of molecular weight). In step ii, the individual fractions are placed in separate columns and the fractionally extracted with a xylene/cellosolve solvent having a constant composition ratio within the range of 60/40W100/0, while the temperature is varied from room temperature to 130°C (fractionation on the basis of chemical composition). The above procedures may be performed in reversed order (ii)→(i).