SysML is a standard language that permits to model systems of any type such as plane, ships and software intensive systems. Software Product Line large scale reuse approach has demonstrated its success. The industry provides benefits in term of cost savings and acceleration of time to maket. The available literature indicates that there have been efforts to enhance the capability of SysML in handling product families. However, these attempts are not yet fully systematic, and there remains a significant amount of work to be undertaken in this area. In this present paper, we deal with the SysML Block Diagram in order to investigate to what extent it permits variability representation and how it can evolve during the system evolution or when agility is needed. We want to capitalize on the knowledge necessary for block diagram extention and evolution and take advantage of knowledge from Product Line domain engineering and application engineering. So, we decide to use an ontology which is an articifial intelligence artifact. An ontology is a powerful mean to represent knowledge and reason about it. Here, we use the ontology to help decision making for Block diagram evolution as well.