An automatic treebank conversion method is proposed in this paper to convert a treebank into another treebank. A new treebank associated with a different grammar can be generated automatically from the old one such that the information in the original treebank can be transformed to the new one and be shared among different research communities. The simple algorithm achieves conversion accuracy of 96.4% when tested on 8,867 sentences between two major grammar revisions of a large MT system. Motivation Corpus-based research is now a major branch for language processing. One major resource for corpus-based research is the treebanks available in many research organizations [Marcus et al.1993], which carry skeletal syntactic structures or 'brackets' that have been manually verified. Unfortunately, such resources may be based on different tag sets and grammar systems of the respective research organizations. As a result, reusability of such resources across research laboratories i...