The behaviour of the thin web plate of two welded unstiffened beams under uniformly distributed vertical load was analysed in this thesis. The difference between beams is in the web thicknesses while the material properties and the remaining geometrical properties are the same. According to a laboratory test, two numerical models were created in the ABAQUS computer program. Laboratory tests are detailed described in this thesis, the dimensions of the beam samples are defined, and the testing procedure and the analysis of the obtained results are discussed. The tests were carried out on two welded thin web plate beams without intermediate transverse stiffeners under uniformly distributed loads. The aim of developing numerical models is to approximate the results of laboratory testing. Numerical models performed two analyses: linear and nonlinear. Buckling eigenvalues of beams has determined using linear analysis while the actual deformation value of the beams has determined using nonlinear analysis (''post-buckling''). The primary goal was to compare the numerical simulation results obtained by the ABAQUS computer program with the results of laboratory tests and to analyse the buckling behaviour of welded thin web plate beams without intermediate transverse stiffeners. Buckling is an instability of structures balance that occurs from compressive loads or stresses. A structure or its components may fail due to buckling at loads that are far smaller than those that produce material strength failure. During the design of thin steel beams, it is necessary to know well its nonlinear properties, buckling problems and the loss of stability of such elements.