Abstract Laser joining with both longitudinal and transverse joining modes is reported for the plain carbon steel C1010 substrates using Fe48Cr15Mo14Y2C15B6 amorphous alloy filler. The high power (2000–3000 W) laser joining at the laser scanning speed of 6 mm/s resulted in complete fusion and full weld penetration of 3-mm thick steel substrates. Formation of fine cellular/dendritic structure with unresolved interdendritic precipitates was observed in the laser welded regions. As the solidification during laser processing occurs with fast cooling rates, the failure to retain amorphous structure in the laser joined regions may be attributed to the compositional changes/redistribution in the molten material due to partial melting of steel substrates. Fine dendritic/cellular microstructure resulted in significant improvement in microhardness of the welded regions as compared to the substrate hardness of 150 HV. The microhardness of laser joined samples was in the range of 750–1150 HV. The microhardness was relatively uniform along the depth of laser welded regions, consistent with the microstructural observations.