This paper presents experimental and numerical analysis of the composite steel-concrete columns. Three columns are tested experimentally. Overall forty-eight FE models are created. Sixteen different models for every experimental column are analysed to evaluate the influence of the different types of imperfections. It was found that the imperfections reduced the resistance of the composite columns by up to 10%. Limiting the geometrical imperfection amplitudes to B/200, the steel profile effective cross-sectional area reduction by up to 23% was observed, while the critical buckling stress was reduced by up to 74%. Expressions for the calculation of the effective cross-sectional area ratio and critical buckling stress are proposed. Keywords: thin-walled steel section, steel-concrete composite column, geometrical imperfections, residual stresses.