Business Process Model and Notation (BPMN) is a graphical modelling language for specifying business processes. Among the open issues existing in the business process development, one of them aims at providing techniques for comparing two versions of a process model. Comparing processes is useful for tackling several problems such as process reconfiguration or evolution, process harmonization or effective search. In this paper, we propose two measures of similarity between two versions of a BPMN process. The first one relies on the syntactic descriptions of the two processes considered as input, whereas the second one focuses on their semantic models. These two measures are complementary and allows users to better understand the differences and similarities between the two processes. Our approach is fully automated by several tools we reused or implemented for this work.