We can understand "Shanghanlun(傷寒論)" and "Wenbingtiaobian(溫病條辨)" which are major books on externally contracted diseases well by making a comparative study of their similarities and differences. After studying etiological causes and characteristics of disease, disease pattern, syndrome differentiation, transmutation rules, following conclusions are derived. While cold is an etiological cause of Cold damage and harms Yang qi, heat is an etiological cause of Warm disease and harms Yin qi. Cold damage and Warm disease have something in common in the respect of damage to fluid and humor and Yang qi. Exuberant heat symptom of Yang brightness disease and lesser yin heat transformation pattern have similar damage to fluid and humor as Warm disease does. Warm disease can reach qi collapse syndrome through damage to Yang qi following fluid and humor damage. In the respect of water qi, as Cold damage makes water-dampness retain easily due to cold congealing, dampness-draining diuretic medicinal and warm yang medicinal are used together. As warm disease damages fluid and humor, yin-tonifying medicinal is used and dampness-draining diuretic medicinal can be used in the case of Warm disease with dampness. In the respect of disease pattern, cold syndromes arise mostly by Cold damage except heat syndrome of grater yang disease, chest bind syndrome, stuffiness syndrome, reverting yin disease and yang brightness disease. Warm disease is classified as pure heat syndrome and heat syndrome with bowel excess, damage to yin, qi collapse or damage to blood.