Abstract: The assembly deformation introduces one of the most important reactivity feedbacks in fast neutron reactor. To evaluate the reactivity caused by assembly geometry deformation, the virtual density theory was applied. In this theory, the virtual material density was changed instead of changing the geometry in modeling the assembly deformation. Compared with existing methods, this theory has better geometric adaptability and better efficiency. A series of verifications show that the virtual density theory has good precision and efficiency in evaluating the assembly deformation reactivity.