The modern trend of gas turbines design is towards lighter, highly efficient,and more compact engines. Such situation imposes on engineers to continuouslysearch for improved and optimum designs. The thesis presented aims at researching possible performance improvements regarding axial gapdistance in transonic compressors. Decreasing the axial gap would result inlighter engines and achieve design goals. The influence of decreasing the axialgap on performance and structure integrity should be throughly analyzed. This thesis work includes numerical investigations on the axial gap distance effect on performance efficiency and related unsteady aerodynamics phenomena. The first one and a half compressor stages of a Siemens Gas Turbine are modeled in ANSYS CFX. Different axial gap models are simulated for differentconfigurations. The steady state solution is obtained to be initialized for transient time marching calculations. Furthermore, the computational cost of transient calculations is reduced through a geometry scaling technique. The unsteady behavior is further analyzed by a Harmonic Balance solver implemented in STAR-CCM+ software, and compared to a reference case transient calculations. The results obtained supports the presence of an optimalaxial gap distance for maximum efficiency in transonic compressors. Further, the harmonic balance method shows good possibilities for cost and time reductions in transonic compressors performance calculations.