In order to explore the dynamics and composition of the Venus atmosphere, characteristics of several types of Venus balloons have been investigated. Vapor of a suitable liquid in the balloon gives buoyancy in the carbon dioxide atmosphere of Venus. In this case the phase transition of vapor to liquid occurs as the temperature changes with the altitude within the Venus atmosphere. Thus, the phase transition of vapor to liquid in the balloon works to stabilize the balloon altitude. A model experiment testing the phase transition balloon in a water vessel with an appropriate temperature gradient and some results obtained with this model experiment are described.