On the deployment mechanism of a large deployable onboard antenna of ETS-VI (Engineering Test Satellite Type 6), flight data are compared with predicted one. The deployment mechanism is thermally suitably controlled by heaters and by MLI (Multi-Layer Insulation) blankets. Such thermal control permits of deploying the main reflector within allowable temperature limits. That mechanism has thermally been evaluated with a mathematical model including thermal resistances between the inner and outer bearing rings. A good accuracy of the proposed method has also been demonstrated from flight data.