A dispersionless X-ray Kα spectrometer is constructed. The experiments were performed using the SILEX-I laser facility. The Kα Ｘ-ray emission from ultraintense laser-plasma interactions is measured with the spectrometer in front of and behind the foil targets. The conversion efficiency from laser energy to Kα Ｘ-ray is obtained. The experimental results show that the Kα yield increases with the laser energy. With a 100μm thick Mo foil, the conversion efficiency is 10-5.