Breast thermography is considered particularly valuable for early breast tumors detection. The fast growing tumor has a higher metabolic rate and associated increase in local vascularization. It will cause the occurrence of some asymmetric heat patterns. Clinical interpretation of a breast thermogram is primarily based on the asymmetry analysis of these heat patterns visually and subjectively. In this paper, a new approach of asymmetry analysis of breast thermograms was proposed. The heat patterns are first segmented with mathematical morphology. The asymmetry analysis is performed both qualitatively and quantitatively according to the extracted features. The abnormality of a breast thermogram is clearly indicated by the features.