The inlining compiler optimization replaces function calls with the body of the called function, thereby removing the overhead of the function call and enlarging the scope for further optimizations. This thesis describes improvements in the existing inline optimization in libFirm, which is an implementation of the fully graph based intermediate representation Firm. The limited functionality of the existing libFirm inliner has been enhanced and completed. Further improvements were made by replacing the inline heuristic, which determines the inline decision and order for replacing a set of function calls. This thesis concludes, that the inline optimization has been improved in several ways. As a result, the runtime of the SPEC CPU2000 benchmark specification could be improved by 4.25%.