Soundness of workflow system is a fundamental requirement in formalizing transaction processing flow. Most current researches convert soundness of workflow net (WF-net) system to liveness and boundedness of its extended workflow net system. The soundness property is studied in this paper from the language viewpoint of the workflow net system. Net language depiction for soundness is presented to reveal the behavior
characteristic of this basic property. Furthermore, synchronous composition and combined composition of the workflow net system is studied. Necessary and sufficient conditions are given for soundness preservation of the two composition systems. This study provides a novel approach to the modeling and analysis for composition in complex WF-net systems.