In this paper, the Joule heat welding of thin Pt wires with different diameters was performed and the current required for successful welding was investigated. The diameter of one wire was 800nm and the others had various diameters of 1, 2, and 5μm. Various combinations of wire lengths were used in the welding experiments. The minimum and maximum currents for successful welding were found to be highly dependent on the length of the 800nm diameter wire. From these experimental results, it was inferred that the highest temperature in the system during welding occurred in some part of the 800nm diameter wire and that the temperature in the larger diameter wire with sufficient heat capacity were almost unchanged. It was also found that the conditions for successfully welding wires of different diameter can be described by a parameter previously proposed for classifying the successful conditions for welding two 800nm diameter wires.