In the present work, 2mm thick plates of a single crystal nickel base superalloy, CMSX-4, were welded using a CO2 laser. The effects of the welding direction deviating from the base metal <100> direction on the microstructure of the weld fusion zones were investigated. In bead-on welding, while the welds with deviation angles of 15 degrees or more showed fusion zone transverse cracks, those with deviation angles within 5 degrees had no cracks and the solidification grains grew epitaxially from the base metal gains except for the bead neck regions, which contained some stray crystals. As deviation angles increased, number of the stray crystals increased. As a result, the major part of the fusion zones can be the single crystal having the same crystal orientation as the base metal when deviation angles are within 5 degrees. The butt welds with the declinations of the crystal orientation of the two base metals up to 10 degrees had no cracks. The bead-on weld and the butt weld joints having no cracks were more than 800MPa, which is almost 80% of that of the base metal, in tensile strength at 1123K.