Software Product Line (SPL) is a development paradigm that targets the creation of software system variants that belong to the same domain. Usually software system variants, developed with clone-and-own approach, form a starting point for building SPL. To migrate software systems which are deemed similar to a product line, it is necessary to detect the com- mon features and variations between a set of software system variants. Reverse engineering the feature model (FM) of an existing system is a challenging activity. FM describes the common and variable characteristics of a product line. In recent years, a lot of work has addressed the extraction of FM from different artefacts. Little work addressed extraction of FM from source code. This paper proposes a general approach to extract initial FM from the object-oriented (OO) source code of a set of software system variants in order to support the migration process from conventional software development to software product line engineering (SPLE). We present an approach to extract features of FM from the analysis of object-oriented source code for a set of software product variants. This approach is based firstly on the definition of the mapping model between object-oriented elements (OOE) and those of FM. Secondly; it uses an identification process exploiting on the one hand Formal Concept Analysis (FCA) as a method for clustering OOE corresponding to the implementation of features and on the other hand Latent Semantic Indexing (LSI) to define a similarity measure on which is based this clustering.