The use of a Langmuir probe to measure the plasma parameters requires that the disturbance caused by the probe be localized, which is the very basis for the interpretation of the probe characteristics.However, the thermal plasma conditions under a practical use could be affected by the insertion of a solid, cold probe severely.In this study, we calculated flowing thermal plasmas with three different sizes of a Langmuir probe into an argon free burning are system and investigated the thermal and flow disturbances caused by the metallic probe.For the computations, we have done PHOENICS, one of commercial CFD codes, to work with user-coded subroutines by FORTRAN for coupling the flow field with the electromagnetic together.From the results, it is found that there is the severe disturbance affecting a large part of the thermal plasma, especially the axial velocity field, by the inserted probe.Also the disturbed region is changed by the different sizes of a probe, and its critical size for a practical proposition to flowing thermal plasmas can be decided.