Abstract: In-plane elastoplastic instability of tapered members under axial loads is studied and a new design method is suggested in which the tapered member is equivalent to a prismatic member with the cross-section at the larger end of the tapered member as compared to the current design code CECS102: 2002 where the smaller end cross section is used. A table for determining the equivalent effective length of tapered member is also given. In addition, the elastoplastic working behaviors are compared between the tapered members and the elastic equivalent prismatic ones, and it is found that the tapered members have a higher ultimate load carrying capacity than that of the elastic equivalent prismatic one.