The circular polarized microstrip antenna in the stainless pipe is analyzed by the finite-difference time-domain (FDTD) method with PML absorbing boundary condition. The microstrip antenna is used as a microwave energy supply element for the in-pipe micromachine inspecting. The numerical results agree well with the measured ones. The circular polarized microstrip antenna in the stainless pipe is analyzed by the finite-difference time-domain (FDTD) method with PML absorbing boundary condition. The microstrip antenna is used as a microwave energy supply element for the in-pipe micromachine inspecting. The numerical results agree well with the measured ones.