The porosity formation mechanism of the laser spot microwelded joints between Pt-Ir alloy and 316L stainless steel wires was investigated by characterizing the weld geometry, morphology and distribution of porosity in cross sections. The porosity is believed to be a result of preferential vaporization of stainless steel alloying elements spread into weld pool. Porosity is also attributed to low solubility of stainless steel alloying elements in weld pool due to its high temperature. In addition, the solidification process is speeded up by high heat conductivity of Pt-Ir alloy as well as characteristics of laser microwelding, and the porosity is formed