Microstructure modification by laser remelting or laser alloying was studied on carbon Ck45 and high speed steels. Laser remelting of Ck45 by overlapping laser tracks led to a great refinement of martensitic structure, especially in the heat affected zone of subsequent laser track. High speed steel (HSS) M2 after laser remelting showed, beside the tetragonal martensite, the diffraction lines of cubic carbides of the M6C and M12C types. Laser alloying of M2 HSS using vanadium carbide (VC) additions caused increasing of eutectic in the interdendritic space, which was accompanied with reduction of the M6C and rising of the MC. M2 HSS laser alloyed with molybdenum carbide (Mo2C) showed formation of the M6C for the hipereutectic compositions while at the highest concentrations of molybdenum, primary dendrites of the M2C and stabilized ferrite were stated. High additions of borides: CrB or VB2; developed formation of the primary borides of blocky type containing a high amount of W, Cr or W, V, respectively. Laser alloying of Ck45 by means of: CrB, VB2 and B4C showed: in the case of CrB an eutectic (alpha) '/M3(C,B)/M2B as well as primary precipitates of the M2B phase for hipereutectic compositions; by adding VB2, the M3B2 and M2B phases were identified experimentally for hipereutectic concentrations; for alloying using B4C, the cellular dendritic structure together with primary borides of the (tau) -M23(C,B)6 phase were stated for hipereutectic compositions. The phase diagrams of M2 HSS + (VC or Mo2C) as well as Ck45 + B4C systems were calculated to predict changes of the constitutions due to laser alloying. Comparison of the solidification structures established experimentally with the calculated phase diagrams revealed a good correlation for the carbides, especially.