Buckling of T-Sections under compression is studied analytically considering the interaction between flange and web. Formulas for the critical stress of the cross-section are presented. Extending the classification philosophy of steel plates to steel cross-sections, the paper obtained the interaction curves between the limiting values of the width-to-thickness ratios of flanges and webs for 4 classes(S1, S2, S3, S4) of the T-section. Based on the interaction curves, it is revealed that the classifications are composed of two regions: one is the traditional, in which flange and web are classified independently; the other is the extended region for which the width-to-thickness ratio of the web is relaxed and correspondingly the flange's width-to-thickness ratio is more stringent to reflect the weakening effect to the flange itself due to providing restraint to the web. For 4 classes of T-section, the limiting values are presented for possible references of codification.