Duty to inform is part of lawyer’s obligations required by contract and law. The failure of such duty is considered as professional negligence and so resulted in lawyer’s civil responsibility. The purpose of this paper is to comparative study of lawyer’s duty to inform his/her client and a third party, also to explore the related cases and liability by searching differences and discovering similarities between Iranian and American legal systems and to reveal the importance of issue so it would be useful to legislature for establishing effective legal rules or modifying the law. In this article, the necessity of pre-contract disclosure and lawyer's duty to consistently keep client informed during client-lawyer relationship is achieved through comparative scrutiny of the two legal systems by descriptive analytical approach, with focus in informed consent in American law and client interest in Iranian law. A lawyer duty to inform includes any information affect client consent and decision making regarding the lawyer itself or legal representation. Despite conflict between the duty to warn a third person and the duty of confidentiality, disclosure appears necessary in cases another person is in significant danger posed by client.