Clean Development Mechanism (CDM), is a market-based mechanism under the Kyoto Protocol. It allows developed countries to implement emission reduction projects in developing countries, to assist their sustainable development; meanwhile, developed countries gain credits which could be used to meet part of their emission reduction targets under this protocol. There is a wide range of various types of CDM projects, among which, wind power projects account for the largest share. Additionality is a key component for CDM projects’ eligibility and to ensure CDM’s environmental integrity. It means that the emission reduction after the implementation of the project is additional to any that would have occurred in the absence of the certified CDM project. For wind CDM projects, the controversy on additionality raised debate and attention internationally, especially the concern if project developers deliberately underestimated Plant Load Factor (PLF) so as to meet the CDM additionality criterion. At the United Nations Framework Convention on Climate Change (UNFCCC) CDM Executive Board 63rd meeting, the UNFCCC Secretariat and the Registration and Issuance Team (RIT) proposed different decisions regarding a request forissuance of a Wind CDM project due to different views on the estimated PLF. The Board discussed the issue andrequested a study on the PLF variations, which is the origin, and also a partial content of this thesis.In this thesis, relevant parameters are firstly defined – parameters such as Estimated PLF, Actual PLF, etc, and to better illustrate the magnitude of the PLF variation, the concept of Relative Variation of PLF is adopted, which is measured as the absolute difference of Estimated PLF and Actual PLF compared with Estimated PLF. Then a database in spreadsheet with all defined parameters of sampled projects is set up on the basis of collected information and calculation. Afterwards an investigation of PLFs and comparison analyses of Estimated PLFs and Actual PFLs is conducted. Considering there are two groups of Wind CDM projects, i.e. small-scale and large-scale; and considering the distributions of projects’ hosting countries, the comparison analyses are then conducted firstly for all projects, and then separated for small-scale and large-scale projects, and lastly separated for projects based on different hosting countries. The final results show that a minority of projects have underestimated PLFs, and a very small proportion of projects, either in all projects, or on different scales or in different hosting countries have underestimated PLFs to a level that is out of acceptable range. Therefore, thestudy concludes that there should be no concern on the PLF issue in Wind CDM projects.