The tool holder is the core component of the highspeed tooling system,and its performance directly affects the reliability of the tooling system.The welded connection of the tool holder body and the horn is the weak part of the tool holder,which is prone to fatigue failure.In order to investigate the fatigue behavior of welded connection of certain ISO20 tool holder for high speed machining，tensile fatigue test for ISO20 tool holder welded specimens was conducted by conventional fatigue tests.The results show that the fatigue life of ISO20 tool holder weld connection increases significantly with the decrease of the maximum load;when the maximum load is 0.15Fb，the fatigue life exceeds two million times;the equation of ΔF-N curve of ISO20 tool holder welded connection under the stress ratio R=-1 is lgN=11.732-5.156lgΔF。