A method for preparing a window member for a flexible display device, the method comprising: preparing a hard coating composition comprising a solvent and a hard coating solid matter that contains photoinitiators, first oligomers, second oligomers, and cross-linkers; applying the hard coating composition on a flexible base member to form a preliminary hard coating layer; drying the preliminary hard coating layer to remove the solvent; and photo-curing the dried preliminary hard coating layer, wherein the molecular weight of the first oligomers is 2,000 to 6,000 and the molecular weight of the second oligomers is 10,000 to 30,000, wherein each of the first oligomers is represented by Formula 2: in which n is an integer selected from 8 to 30; R 1 is a hydroxy group, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms for forming a ring, an alkoxy group, a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, or a photoinitiated reactive group; at least one of the plurality of R 1 (s) is a photoinitiated reactive group; and R 2 is a hydroxy group, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms for forming a ring, an alkoxy group, a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, or a photoinitiated reactive group; and each of the second oligomers is represented by Formula 3, in which m is an integer selected from 46 to 150; R 3 is a hydroxy group, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms for forming a ring, an alkoxy group, a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, or a photoinitiated reactive group, at least one of the plurality of R 3 (s) is a photoinitiated reactive group; and R 4 is a hydroxy group, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms for forming a ring, an alkoxy group, a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, or a photoinitiated reactive group. preparing a hard coating composition comprising a solvent and a hard coating solid matter that contains photoinitiators, first oligomers, second oligomers, and cross-linkers; applying the hard coating composition on a flexible base member to form a preliminary hard coating layer; drying the preliminary hard coating layer to remove the solvent; and photo-curing the dried preliminary hard coating layer, wherein the molecular weight of the first oligomers is 2,000 to 6,000 and the molecular weight of the second oligomers is 10,000 to 30,000, wherein each of the first oligomers is represented by Formula 2: in which n is an integer selected from 8 to 30; R 1 is a hydroxy group, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms for forming a ring, an alkoxy group, a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, or a photoinitiated reactive group; at least one of the plurality of R 1 (s) is a photoinitiated reactive group; and R 2 is a hydroxy group, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms for forming a ring, an alkoxy group, a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, or a photoinitiated reactive group; and each of the second oligomers is represented by Formula 3, in which m is an integer selected from 46 to 150; R 3 is a hydroxy group, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms for forming a ring, an alkoxy group, a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, or a photoinitiated reactive group, at least one of the plurality of R 3 (s) is a photoinitiated reactive group; and R 4 is a hydroxy group, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms for forming a ring, an alkoxy group, a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, or a photoinitiated reactive group.