An experimental investigation was performed to analyze the characteristics of a two-phase ejector in carbon dioxide operated heat pumps. Firstly, this paper shows the way how the ejector behaves in the heat pump cycle. The influence of the diameter and the length of mixing tube on ejector performance is studied. Mixing tube diameter has a great influence on pressure recovery according to the conservation of momentum. Mixing tube diameter also has an influence on the acceleration of suction flow before mixing tube entrance, which changes pressure recovery, too. When the mixing tube is too long, the ejector performance slightly becomes worse because of frictional loss. When the mixing tube is too short, the ejector performance drastically deteriorates, because the driving and suction flows pass the mixing section before they are mixed sufficiently. The highest ejector efficiency shown in this paper is 0.46, which corresponds to 23-24% of COP improvement.