The diode laser brazing of heat-resistant alloys with precious brazing filler metals has been conducted using the tandem beam consisted of preheating beam and main brazing beam. The 1mm thick plates of Inconel 600 and A286 alloys were butt-brazed with the 0.5mm diameter Au-18%Ni, Ag-10%Pd and Ag-21%Cu-25%Pd filler metals using a brazing flux. The sound butt joints which were free from brazing defects such as porosity and lack of penetration could be obtained at the brazing clearances of 0.1-1.5mm. Fracture strength of braze joints using Au-Ni and Ag-Cu-Pd filler metals was comparable to the base metal strength at any brazing clearance between 0.1-1.5mm, whereas that using Ag-Pd filler metal increased with decreasing the brazing clearance and attained about 70% of the base metal strength at brazing clearance of 0.1mm. The computer simulations of the braze metal flowing and the base metal erosion suggested that the preheating effect during the tandem beam brazing resulted in the superior brazability at the narrow-gap as well as wide-gap brazing attributed to the improvement in the wetting, spreading, infiltrativity and the erosion resistance of melted filler metal.