As an effective reuse approach, product line engineering (PLE) is used to develop a number of applications by instantiating a reusable core asset. Current research works mostly center on domain engineering, it is yet to define how to instantiate core assets systematically and to validate the instantiation process. We believe that formal specifications have a great potential in defining detailed and precise instantiation process. In this paper, we first define the key elements of core assets, and show how to formally specify core assets Object-Z. Then, we present rules for defining the instantiation process and relevant constraints. Using the proposed formal framework, the task of instantiating core assets can be carried out precisely and correctly, yielding high quality software development.