S960 and S700 are two types of new high strength low alloy (HSLA) steels recently developed by Tata Steel, which are used for heavy lifting equipment. Little is known on their welding characteristics with high-energy beams. In this study, single pass autogenous laser welding (ALW) and multipass ultranarrow gap laser welding (NGLW) were carried out using a 16 kW IPG fiber laser to examine the weldability of these HSLA steels up to 13 mm in thickness. It was found that cracking in the welded joints could be avoided. It is beneficial to use horizontal (2G) welding position to avoid melt sagging when welding thick section S700 steel plates in a single pass ALW process. The fast cooling rate for the ALW of 8 mm thick S960 steel and ultra-NGLW of both the 8 mm thick S960 and 13 mm thick S700 steels causes the generation of hard martensite in the fusion zone (FZ) and heat affected zone (HAZ). This could strengthen the laser welded HSLA steel joints, but would deteriorate the toughness of the welded joints. A relatively higher heat input was used to weld the 13 mm thick S700 steel using a single pass ALW process, which causes the formation of bainite in the FZ, while hard martensite and bainie were formed in the HAZ. All the ALW and ultra-NGLW S960 and S700 steels presented almost the same tensile strengths as the base materials and also retained the elongation of the laser welded specimens, quite unexpected.