Abstract: The inner flow field in the 45°sharp-edged T type junction of the axial section was measured on the pressure loss of the exhaust system by Particle Image Velocimetry (PIV).Char-acteristics of flow type 6 were obtained at different mass flow rate ratios and Mach numbera.The pipe area in each branch of the tested 45°junction remained constant,D =50mm.The axes in all branches were coplanar and the corners at the point of intersections between lateral branch and main branch were sharp edged.The experimental data were measured for the junction with vari-ous mass flow rate ratios (q =0,0.25,0.5,0.75 and 1)and Mach number in common branch (Ma=0.13,0.31 and 0.59).Comparatively analysed the data of the velocity field and streamline pattern.Conclusion were drawn out that flow contraction occurred in downstream of the junc-tion.The feature of the flow field was related to the Mach number in common branch and the mass flow rate ratio between branch and duct in type 6.As the mass flow rate ratio of branch and duct increased,the turbulent region was enhanced and pressure loss enlarged.Mach number would also change the flow characteristics and pressure loss.The variation in the total pressure loss coefficient with Ma was fairly small in the range of 0.13 up to 0.31.However,along with the Mach number increased to 0.59,the coefficient increased more rapidly.