Based on étendue, relative illumination of projection lens is analyzed. With consideration of the inner intensity distribution and angular distribution of light source, the intensity distribution vs étendue of facular emitted from lamp together with reflector has been deduced. Moreover, the intensity vs angular distribution of facular on LCD panel, which is far different away from Lambert distribution, has also been obtained by étendue transfer. Under the analysis, calculation method for relative illumination is revised, and relative illumination of LCD projection lens is calculated. By experiment, the new method based on étendue has more accuracy in relative illumination calculation for LCD projection lens.