In this study bead-on-plate tests on 3 mm-thick Ti6Al4V sheets were performed using a Yb:YAG thin disk laser in continuous mode to evaluate the effects of focus position on the bead characteristics. Among the various solid-state lasers, the disk laser has better beam quality than conventional rod geometries thanks to the absence of thermal lensing effect. An experimental plan was conducted using nine focus positions with constant power and welding speed. The beads were characterised in terms of geometry, defects, hardness and microstructure. This work is a preliminary study to select the best values of the focal position for subsequent butt welding tests. Analysing the results obtained, the best value of defocusing was -4 mm because, among full penetrated beads, it showed lower undercut and excessive penetration, lower melted area and smaller grain size in the melted area.