The maJor alm of this paper is the research of finite deformatlOn of orthotroplc elastlcplasticity for a round bar under tenslon. Meanwhile , on the basis of multiplicatlve decomposition concept, a finite element method suitable to calculating the finite orthotropic elastlc-plastic deformatlOn has been suggested. The slmulated results of the orthortropic finite plastic deformatlOn of a bar made of orthortroplc plastlc material under large stram tenslOn show the shape of the necking sectlOn of the bar IS elliptic , and there are very evident differences m mechanical behavlOr and in defferent directlOnal necking deformatlOn when the tenslOn is in different onentatlOn.