In order to satisfy the requirements of productivity and quality, in recent years, LASER welding processes of certain grades of steels mainly thin sheets assemblies or tailored welded blanks (TWBs) have given many successful advantages in manufacturing engineering. This welding process of in particular makes it possible very easily to joint the steel parts in thickness and grade identical or different in many industrial applications (car industry, pipe lines, tubes, etc.). Additionally, interstitial free (IF) steels with very low C and N contents have been successfully developed in order to perform specific or complex deep drawing operations in the automotive industry. Major developments of TWB by means of LASER welding in this area have also occurred. Certain grades of IF steel are particularly suitable for deep drawing operations of TWB due to high r values (Lankford ratio) [Grumbach, M., Sanz, G., 1972. Ductile-brittle tests by impact tensile test (ITT). J. Metall. 2, 145-164; Bayraktar, E., Kaplan, D., Buirette, C., Grumbach, M., 2004. Application of impact tensile testing to the welded thin sheets. J. Mater. Process. Technol. 145 (1), 27-39; Bayraktar, E., Kaplan, D., Hugele, D., Jansen, J.P., 2004. Evaluation of pipe line LASER girth weld properties by Charpy and impact tensile tests. J. Mater. Process. Technol. 147 (11), 155-162; Bayraktar, E., Kaplan, D., Grumbach, M., 2004. Application of impact tensile testing to the spot welded sheets. J. Mater. Process. Technol. 153, 80-86]. However, because of the narrowness of the weld bead and important hardness gradient, determination of "toughness" (definition of a criterion of the "ductile/brittle" transition during the impact test) of the welded connection (weld bead and heat affected zone) confirm very delicate in LASER welding. The main objective of this paper was to make a comparative study for microstructural and mechanical-toughness characterisation of the steel assemblies, base metal or welded by LASER. For that a procedure of test specifically adapted was used for welded joints (t < 4mm). The first part of this study consider different steel grades such as a grade of the FE360BFN steel (t = 4 mm) and a grade of the interstitial free steel (IF-Ti, t = 0.8 mm) and HR60 (t = 2.5 mm) welded by LASER that largely used in the automotive industry. (C) 2007 Elsevier B.V. All rights reserved.