Assessment is the foundation of transition. This article described the meaning and functions of transition assessment firstly. Then, the contents and administration of transition assessment, the contents and development of transition planning were presented. Next, the article focused on discussing the development and contents of a transition skills assessment instrument for senior high school students with disabilities. Finally, the application of this transition assessment instrument on developing individualized transition planning was illustrated, including the following seven steps: (a) understanding the background information of the case; (b) administering a Transition Skills Assessment instrument for senior high school students with disabilities; (c) synthesizing assessment results into a profile; (d) identifying the need of advanced assessment; (e) analyzing transition skills of the case; (f) developing individualized transition planning; (g) implementing and evaluating individualized transition planning.