A continuously variable power transmission will provide a passenger comfortable cruising and powerful acceleration as an automotive propulsion system. A half-toroidal traction drive is one of the mechanical CVTs and has a low spin moment in the EHL contact area. In this paper, theoretical approaches to the prediction of speed and torque transmission efficiencies have been made. The speed ratio changing mechanism which swings the power roller by side slip force is also discussed. Theoretical results are compared to test results of a prototype of nominal input torque capacity of 120 Nm, rotational speed of 5000 rpm and speed ratio range of 0.4 to 2.5.