In binary phase diagram there are many stable compounds. Hereby a new method for calculating activities of compounds from binary phase diagram involved stable compounds is presented. Based on the available formula for calculating activities in binary phase diagram, the activities of the two pure components are gained. And the balance constant K~0 can be obtained from mass action law. Thereby, the activities of stable compounds are obtained from the activities of the two pure components. By using this method, the activities of stable compounds in In-Sb system are obtained. It proves that this method is feasible.