Circular arc recognition is a difficult problem that is not entirely solved in engineering drawing recognition and understanding research field. In this paper, some concepts of global recognition method and patch structure are introduced, and a circular arc recognition algorithm is provided. The algorithm matches several patches with two patterns to decide the seeds, and then by tracing the other patches, it converts these patches into circular arcs. Examples are given, which show that the algorithm has effectively solved a key problem of recognizing drawings are given.