The surface defects on MBEgrown HgCdTe films on GaAs substrates were studied, and there was a type of defects that relates to the impurities from mercury source. Faces and crosssections of the defects were observed by using SEM, and the component analysis was performed by using EDX.Two experiments were designed: one was growing CdTe film on CdTe/GaAs substrate with high temperature, after the substrate with low temperature exposed on Hg flux in 20min; the other was growing CdTe film on CdTe/GaAs substrate with high temperature, being exposed on Hg flux all along in the CdTe growth. The defects that were similar in shape and distribution to those on the surfaces of HgCdTe films appeared on surfaces of CdTe films after two experiments, and the defects on the surfaces of two CdTe films were observed by using SEM. By comparing the defects on the surfaces of two CdTe films with the defects on the surfaces of HgCdTe films, we finally confirmed the fact that the impurities from mercury source resulted in the nucleation of this type of surface defects on HgCdTe films.