Abstract. An outerplanar graph is a planar graph which can be embedded in the plane in such a way that all of vertices lie on the outer boundary. Many semi-structured data like the NCI dataset having about 250,000 chemical compounds can be expressed by outerplanar graphs. In this paper, we consider a data mining problem of extracting structural features from semi-structured data like the NCI dataset. For this data mining problem, first of all, we define a new graph pattern, called a block preserving outerplanar graph pattern, as an outerplanar graph having structured variables. Then, we present an effective Apriori-like algorithm for enumerating frequent block preserving outerplanar graph patterns from semi-structured data in incremental polynomial time. Lastly, by reporting some preliminary experimental results on a subset of the NCI dataset, we evaluate the performance of our algorithms.