The purpose of the current study is to investigate how school based curriculum development is unfolding. To understand the practice, the author has used a case study of the school based curriculum innovation to study the decision making process of the principal. The current study adopts the theoretical framework of Stufflebeam's evaluation model which covers 4 components including Context, Input, Process and Product to investigate a case study of a school based curriculum innovation in a Hong Kong primary school. The study focuses on the rationale and the conditions that the principal of the school has considered in the process of making decisions at the initial stage of its conception (context), at the second stage of its development and preparation (inputs), at the third of its implementation (process) and at the fourth stage of its realization with outcomes (product). This paper has used the CIPP model to investigate the underlying rationale of the decisions made by a Hong Kong principal on curriculum objectives, on preparation of teachers, on organization of resources, on the implementation of the program and on the assessment of the outcomes of the innovations. This paper has made it explicit the processes of the decision making and the meanings behind decisions made at every stage of the development of the curriculum innovation and provide a clear evaluation framework. The case curriculum innovation is called Astronomy, Sciences and Technology (AST) which is a newly integrated subject in the formal curriculum structure in Hong Kong. This is a curriculum innovation initiated by the new principal of a primary school in Hong Kong though she has 15 years of experiences as a principal. She is also considered an excellent principal providing mentorship and guidance to other new principals. At the context stage, this principal identified the student needs and how this information informed planning. At the input stage, she developed the lessons and learning materials and worked with teachers. At the process stage, she focused on developing teacher capacity in teaching. At the product stage, she collected information and results which become essential feedback for formative improvement. There are three conclusions: 1. Using the CIPP model, we are able to identify the key features of the decision making processes and contents of the principal in the development of the AST subject. A flow chart of the process is used to illustrate vividly the cycles of decision making; 2. Revealing the close inter-relationship between leadership, curriculum development and evaluation; the principal's support and her commitment have enhanced the school based curriculum development and also developed teachers' professional leadership practices. The comparative autonomy that the Hong Kong principals enjoy in employing teaching staff and resources is showing a greater freedom in curriculum decision making; 3. Besides the Government's support in school based curriculum development, the initiatives and commitments of the principal all contribute to the teacher professional development in school based curriculum development. The leadership of the principal and her continuous efforts in curriculum development has been very important to the success of the school based curriculum development.

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