The problem of the Helmand River has constantly been considered as the main cause of struggle between Iran and Afghanistan over the recent century, and it has not yet completely resolved. The problem was commenced in 1872A.D./1289A.H. with the arbitration of Goldsmid. During the age of Rezakhan’s ruling, negotiations began in 1310 which resulted in signing of the 1317 Treaty. The Iranian government and national council assembly as well as the Afghan government approved the treaty and the additional announcement, while the latter was rejected by Afghanistan’s national council assembly. Although there exists a variety of information respecting the issue of the Helmand River in Rezakhan’s era and some aspects of this question have been considered in the research literature, it is highly required to conduct a comprehensive, complete and documented research in this regard. This article, which is based on the unpublished documents of Iran’s foreign ministry, offering a meticulous, substantiated and abridged description with regard to historical background of the issue and expounding its various facets, is devoting effort to find an answer to the question that why and how the problem of the Helmand River did not lead to a decisive conclusion in the mentioned period. A multitude of reasons were implicated in not finding a solution to the problem of the Helmand River, among which Afghans’ suspicion and their lack of commitment to the exploitation of the Helmand River over Kamalkhan’s dam is of dramatic significance.