Tree structures are used extensively in domains such as XML data management, web log analysis, biological computing, and so on. In this paper we introduce the problem of mining frequent sequential trees in a large tree sequence database. We present a framework for mining frequent sequential trees in a so-called tree sequence database. Basically, this framework employs a transformation-based approach which converts the sequential tree mining problem into a traditional sequence mining problem. Our approach firstly mines frequent trees in the tree sequence database. Secondly, we perform a database transformation by means of tree-containment computation to generate a sequence database. Thirdly, after the transformation, frequent sequence patterns can be mined in the newly created sequence database using a conventional sequence mining technique. Finally we perform an inverted transformation process on the output of sequence mining to obtain sequential tree patterns. Experimental results on synthetic datasets show that the proposed framework is both effective and efficient in finding frequent sequential trees in a large tree sequence database.