A PhD thesis proposal is an extremely important document, and much thought and planning should go into crafting this document. Its immediate purpose is to secure the agreement of your thesis committee to allow you to pursue your research, but it has very long-term implications: not only will the research take several years to complete, it will have a major impact on your search for grant funding, post-doctoral positions, and the competition for a tenure-track position. Writing a clear and effective thesis proposal is the first step you will take on your way to a career in research.
Describe the theoretical framework for the dissertation. This section describes the foundations of the research, especially if the thesis is heavily indebted to a particular approach to a topic, or if it tests the validity of a given theory.
Describe the research problem itself, placed in this theoretical framework. You may choose to include the principal studies that are relevant to your research proposal, although a fuller literature review will be included below.
Describe the hypothesis. This is basically your statement of what you believe the research might indicate. You should have some preliminary basis for this statement, in order to demonstrate that a more thorough investigation is merited.
Describe the purpose of the study. This is perhaps the most important section of the PhD thesis proposal: why should this study proceed? A dissertation at the doctoral level is intended to add to the body of world knowledge; will this study achieve that goal? You should show how it will it change the direction of current research, confirm current research in a novel manner, or in some other way contribute in a positive way to what we know about the world.
Demonstrate a solid grasp of the parameters of the literature to be reviewed. This need not be exhaustive for the proposal stage of the dissertation, but the major sources should be identified.
Describe the methodology of the study. This is especially important for quantitative proposals in the sciences and social sciences.
Describe the limitations of the study. It may seem like your dissertation is incredibly huge in scope, but every research project has its self-defined limitations.
Conclude with the significance of the proposal. This should be a briefer statement of the ideas described earlier in the section describing the purpose of the study, but with a broader focus. Why would the conclusions of this dissertation matter? Would they deepen our understanding of a topic in a major way, or would they lead to some material benefit for humanity?
Attach any relevant appendices, including a preliminary bibliography, samples of survey instruments, and the like.