a method of determining an idle stop and go ventilation condition may include an isg restriction early step that calculates an expected external air temperature through modeling using information acquired from a vehicle and execute an isg logic to determine whether isg entrance is possible based on a current ventilation condition of the vehicle , an isg restriction proceeding step that detects an voltage of an air - conditioning system , an isg restriction ascertaining step that determines whether an isg entrance condition is satisfied , wherein the isg entrance condition includes comparing the expected external air temperature through modeling with a specific temperature and the voltage of the air - conditioning system with a specific voltage , and an isg entrance step that controls an engine by performing the isg entrance when the isg entrance condition is satisfied .