In this paper, we propose a calculation method for the thermal transmittance(U-value) of horizontal sliding windows. We proposed a calculation method of the U-value of casement windows in our previous paper, and reported that the difference from the test value by hot box methods was within 10%. A standard calculation method of the U-value of windows and doors is presently being worked upon by ISO. However, there are some unique features of windows manufactured in Japan, and there is an argument that calculation methods so far proposed cannot be applied. A horizontal sliding window is the most general type of window in Japan, but the details of the sash are different from most casement windows in Europe and America. In addition, there are many wooden sashes in Europe, whereas in Japan aluminum and PVC tend to be used. In the first section of this paper, we described the difference between sliding windows and casement windows from the viewpoint of heat transmission. It was seen that the margin of error grew when previous methods of calculation were applied to sliding windows. So, we proposed an improved calculation method. First, we classified a wide range of manufactured sliding windows by material, the structure of frame and glazing type. From there we selected 25 and comparisons were then made with the test values. With these comparisons the difference between the test values and the calculation method values was within 15% except three windows.