The property of soft X-ray generation from Xe clusters irradiated with intense lasers was studied. The wavelength, the pulse width, and the pump energy were widely varied. Through this survey, even with the comparatively low-density Xe jet of <5 x 10(18) cm(-3) average atomic density, the highest conversion efficiency of over 10% from laser energy to soft X-ray (5-18 nm) was obtained with a femtosecond KrF laser pulse, where a 4pi source was assumed. (C) 2002 Elsevier Science B.V All rights reserved.