Public private partnership has been used extensively in both developing and developed countries in delivering various types of projects, ranging from transportation projects, health care facility and power plants. The high speed rail projects which implemented by public private partnership in recent years has faced with difficulties. This paper aims to develop an identification and assessment method for critical risks of public private partnershiphigh speed rail project in Iran. In this research at first, risks of public private partnershiphigh speed rail projects are identified by integration of extensive literature review and Public private partnership high speed rail project experience in Iran and other countries. In the next step, the identified risks are assessed by questionnaire survey and multivariate statistical analysis. Analytical investigations show that the number of critical risks in high speed rail projects and their mean importance is higher than that of general public private partnership projects. In this study in addition of confirming pre identified risk groups consisting political and financial groups, two major groups of initialisation and contractual risks specific to high speed rail projects are identified and two groups specific to developing countries named circumstances and private risks are proposed. Because of insufficient studies and lack of experience in developing countries about transportation mega projects via Public private partnership this study can be a helpful reference for public private partnership implementation of high speed rail mega projects in developing countries.