The effects of incubated nucleation treatment with adding trace Au or Sn on the solidification microstructure of Au-Sn eutectic alloy Au20Sn were investigated. The results indicate that compared with no incubated nucleation treatment, the solidification microstructures of Au-Sn eutectic alloy are modified greatly by incubated nucleation treatment with Au or Sn. The structure, the morphology and the size of the primary phase are also changed. The primary phase transforms into d-AuSn phase with the shape of rod or egg of uniform distribution and small size from conventional z ’-Au5Sn phase with coarse dendrite of non-uniform distribution. The primary d-AuSn phase in the solidification microstructure becomes more uniform by adding trace Sn than by adding trace Au. Meanwhile the eutectic structure is also refined by the incubated nucleation treatment.