The numerical analysis of laser welding process with consideration of analytical methods determining the phase transformations and mechanical properties of welded joints are presents in this paper. The analytical CCT diagram and final structural composition of S355 steel are presented. The empirical relations presented in the paper are determined by chemical composition of investigated steel and cooling rates between temperatures 800-500^oC (t_8/5). Phase composition and mechanical properties of each structure of welded steel are calculated in weld and HAZ on the basis of analytical methods. Laser welded flat is used in numerical simulations performed in ABAQUS. Different mathematical models of volumetric welding source are used in calculations. Temperature field, shape and size of melted zone for chosen points in the cross-section of the joint are determined on the basis of numerically obtained thermal cycles.