Building a high-quality bioinformatics workflow needs not only good biology knowledge, but also extensive knowledge of existing online bioinformatics services. To facilitate this task, we present a novel approach of automatic workflow composition in SIBIOS system. Our approach regards the task of workflow composition as a planning problem, and uses Graphplan as the solution planner. We extended Graphplan to allow users to specify particular services that they want to use in the workflow, as well as the order those services are used. Automatically generated workflows are used as a baseline for researchers, to be refined to meet their needs. This paper describes the theoretical design and practical evaluation of automatic workflow composition in SIBIOS.