This paper presents a new method integrated in Bell Canada software acquisition process to assess software. This paper is focused on the assessment of the understandability of the inheritance graph of object oriented software. The method is based on metrics and graphical illustration of the inheritance graph. A technique to decompose the inheritance graph of an object oriented software into sub-graphs using metrics is described. Metrics are also used to identify complex sub-graphs. On the selected graphs, a technique to improve the understandability of the graphical illustration representing the inheritance graph is also described. This technique is based on extracting the main tree on the inheritance graph. A C++ case study containing 1080 classes is presented on which this assessment method was applied.