Traditional Software Development Life Cycle (SDLC) is based on informal specification of requirements which are often ambiguous or inconsistent and informal development methods. This ambiguity leads to unverifiable requirements especially when developing critical systems. In this paper, we propose applying Formal Methods (FMs) to SDLC including requirements’ analysis, specifications derivation, and system designing, and validation. The proposed SDLC approach is referred to by Formal Transformed software Development Life Cycle (FTSDLC). Since FMs based on mathematics and logic, we expect our model to work successfully in developing critical systems.