In this paper, first, we propose Kaucher arithmetic for interval number and then solve the system A2n×nXn×1=b2n×1 where aij= [ai,j,`ai,j] and xj= [xj,`xj] and bi= [bi,`bi] based on Kaucher arithmetic such that ai,j .`ai,j ³ 0. Then, we extend this method by use of a- level set for fully fuzzy system of A~2n×nX~n×1= b~2n×1.