The study focuses on the heat transfer coefficient (U-factor) of a window with a long-wave high-reflectivity venetian blind (LWHRVB). Two side-by-side hot boxes are built in a room for testing the window U-factors. The tested window systems are 100% glass area without frames, which are equivalent to the center-of-glass of common windows. The slat of venetian blinds is made of plastic with aluminum film glued on each side, and the long-wave reflectivity is 0.93. The two types of window systems tested are the single glazing and the double glazing with the LWHRVB, respectively. The effect of slat slope-angles of venetian blinds on U-factors is also studied. The empirical equations of the center-of-glass U-factors are derived from the experimental data. The corrections for effects of window frames and outdoor wind velocity are made thereafter. The corrected empirical equations can be easily used to estimate the U-factor of the practical window with the LWHRVB. (C) 2000 Elsevier Science Ltd. All rights reserved.