Inappropriate preparation of respiratory gases is associated with serious complications during mechanical ventilation. To develop a temperature monitoring system of respiratory gases within the endotracheal tube, four newborn piglets were studied using an ultra-rapid-response thermometer attached to the closed endotracheal tube suction system. Respiratory gas temperatures were monitored at the mouth-corner level of the endotracheal tube using three thermocouples (Tairway, inserted into the endotracheal tube via the closed suction system; Ttube_centre and Ttube_wall, embedded within the endotracheal tube 0.5 mm and 1.6 mm from the tube wall, respectively). Univariate analysis showed that inspiratory Ttube_centre and inspiratory Ttube_wall were positively correlated with inspiratory Tairway (both p < 0.001). Multivariate analysis showed the dependence of inspiratory Tairway on inspiratory Ttube_centre and Ttube_wall and deflation of endotracheal tube cuff (p < 0.001, p = 0.001 and p = 0.046, respectively). Inspiratory gas temperature within the endotracheal tube can be monitored using a thermometer attached to the closed endotracheal tube suction system. Our system, with further validation, might help optimise respiratory gas humidification during mechanical ventilation.