The welded structures necessarily having welding residual stresses in their members, the influence of residual stresses on the instability problems is very important and has been studied by the author^{(1), (2), (3)}. In this paper, the influence of residual stresses on the torsional-flexural buckling of thin open walled columns was mainly discussed. The general theory was developed and typical examples were illustrated. The tests of columns with cruciform cross-section were refered to show the experimental verification. The following was found : (a) The influence of residual stresses on the buckling strength was remarkable in case of elastic torsional buckling as well as in case of plastic torsional buckling. (b) The combined torsional flexural buckling load was well approximated by the torsional buckling load or flexural buckling load, whichever the lower. (c) A fair agreement between the test results and calculation was obtained.