Research based on a treebank is active for many natural language applications. However, the work to build a large scale treebank is laborious and tedious. This paper proposes a probabilistic chunker to help the development of a partially bracketed corpus. The chunker partitions the part-of-speech sequence into segments called chunks. Rather than using a treebank as our training corpus, a corpus which is tagged with part-of-speech information only is used. The experimental results show the probabilistic chunker has more than 92% correct rate in outside test. The well-formed partially bracketed corpus is a milestone in the development of a treebank. Besides, the simple but effective chunker can also be applied to many natural language applications.