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 two different sizes of a Langmuir probeinto an argon flee burning arc system and investigated the thermal and flow disturbances caused by the metallic probe.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.