Titanium (Ti) metal and its alloys are desirable materials for ship hulls and other ocean structuresbecause of their high strength, corrosion-resistance and light weight properties. And light weight and corrosion-resistant aluminum (Al) is the ideal metal for shipbuilding. The joining of Ti and Al dissimilar metals isone of the effective methode to reduce weight of the structures. Ti and Al have great differences in materialsproperties, and intermetallic compounds such as Ti3Al, TiAl, TiAl3 are easily formed at the contacting surfacebetween Ti and Al. Thus, dissimilar welding and joining of Ti and Al are considered to be very difficult. However, it was clarified that ultra-high speed welding could suppress the formation of intermetallic compoundsin the previous study. Results of tensile shear strength increases with an increase in the weldingspeed, and therefore extremely high welding speed (50 m/min) is good to dissimilar weldability for Ti andAl. In this study, therefore, full penetration dissimilar lap welding of Ti (upper) - Al (lower) and Al (upper)- Ti (lower) with single-mode fiber laser was tried at ultra-high welding speed, and the microstructure of theinterface zones in the dissimilar Al and Ti weld beads was investigated.