This doctoral thesis focuses on the influence of the shielding gas and newly developed metal transfer mode (CBT – Controlled Bridge Transfer) on the HSLA API 5L X80 weld characteristic. The introduction includes detailed literature overview on the subject of the TMCP steel production, MAG welding process (with emphasis on the newly developed CBT welding process) and effect of the shielding gas on the welded joint characteristics. Goals and hypothesis are also defined in the introductory part of the thesis. Experimental part of thesis specifies the complete welding procedure of the workpieces, conduction of the samples testing and finally the analysis of the obtained results. The entire welding was conducted according to the central composite experiment design, face centered (CCF), which resulted in 22 runs with 3 replicates in the center. The design consisted of 3 factors varied over 2 and 3 levels. Out of the 3 varied factors, 2 of them are numeric (wire feed speed, m/min and CO2 content in the shielding gas, %) and the third one (metal transfer mode) is categorical. For each run from the experiment design a sample from the butt welded joints were prepared in order to conduct chemical and microstructural characterization. Analysis of the chemical characterisation provided information on the exact chemical composition of the weld metal, heat affected zone and base metal as well as of inclusions. Also, microstructure of the welded joint was analysed in order to get information of the portion of different constituents. In the end micro hardness of the each individual phase and weld pass was measured. Entire experimental work was conducted in Laboratories of the Faculty of mechanical engineering and naval architecture, University of Zagreb. All of the gathered data on the chemical and microstructural composition of the weld metal were statistically analyzed with the Design Expert software program. As of the result of the analysis, mathematical models were defined from which the influences of the varied parameters were defined and their interactions on the weld metal characteristics were observed. Also, Monte Carlo Simulation was used in order to estimate the influence of the input values variations on the properties of the microstructural and chemical analysis results.