The main objective of this thesis is to investigate the near-stall endwall flow structures of the gap between the blade and the casing on the performance and the rotor stability of a transonic compressor rotor with and without high-speed endwall air injection. Due to the complexity of leakage flow behavior and vorticities at blade tips and laboratory limitations, this study is carried out using numerical methods via ANSYS-CFX software. The model used in all simulations is NASA Rotor 67, a transonic axial compressor rotor.This study focuses on tip flows, examining the flow structure at the blade tip for different blade tip clearances in both smooth casing and injection cases. In the studies conducted, numerical results are compared with available experimental data at stable near-stall operating points to evaluate the results and ensure the accuracy of the simulation performed. The results had a good agreement with the experimental data. Initially, the effect of changing blade tip clearance sizes on compressor stability and performance is investigated, with numerical results indicating an inverse relationship between overall performance and blade tip clearance size in the compressor rotor. Then, high-pressure continuous air is injected onto four different blade tip clearances at two different air injection velocities and three different air injection angles at the same injection location on the casing. High-pressure air injection onto blade tip clearance can also reduce compressor stability sensitivity to changes in blade tip clearance and increase overall pressure ratio, although this action causes a slight decrease in rotor efficiency. Increasing clearance during injection increases stability while reducing efficiency loss from air injection. Results show that changing clearance sizes in the smooth casing doesn’t change stall type; it starts from walls but changes from wall to blade stall with endwell injection. Also, results indicated that by increasing the injection velocity and injection angle, the rotor reached a higher stability range, especially at higher tip clearance sizes. Key words: Tip clearance size; tip leakage flow; tip vorticities; endwall air injection;