Abstract: A thermal flask has a body and a vacuum solar heating device. The solar heating device is detachably mounted on an opening defined in the hollow body. The vacuum solar heating device has a transparent tube, a connecting collar and a solar energy absorber. The tube has an inner wall, an outer wall, a closed end, an open end and a vacuum chamber defined between the inner and outer walls. The connecting collar is mounted on the open end of the tube and has an inner thread screwed onto an outer thread formed around the opening in the hollow body. The solar energy absorber is mounted in the tube to absorb solar energy and radiant heat. Accordingly, the vacuum solar heating device can heat the liquid in the flask, and the temperature of the liquid will be maintained for a desired long time while traveling.