Abstract: It is well known that the single line array sonar can't identify whether the target come from left or right. Multi-line array provides a possible method to solve the left-right ambiguity problem.The performance of left-right resolution for twin-line array is described in this paper.It is shown that the suppression ratio,which is defined as the ratio of the response in the steering direction to opposite symmetrical direction,can be as an index for evaluating the ability of solving left-right ambiguity.The characteristics of suppress ratio is discussed.The theoretical expression for suppression ratio and some numerical results are illustrated.The result of theoretical analysis can be used in design of twin-line sonar.