The invention of high-power ultra short pulse lasers has opened way to investigations aimed at creation of a new type of bright x-ray source for different applications including material science and time resolved x-ray diffraction for biology. The conversion efficiency of the laser energy incident onto a solid target into the x-ray emission depends on many factors, including the temporal profile of laser pulse. We report here the results of our theoretical and experimental investigations of the line x-ray emission from solid targets irradiated by ultra short laser pulses. The parameters of laser pre-pulse are optimized in order to get the maximum laser energy conversion into the emission in the selected x-ray line.