A nanometer-scale focused spot of hard X-ray can be achieved by multilayer Laue lenses (MLLs). X-ray propagation in MLL is analyzed using dynamical diffraction theory. The focusing performance of tilted MLL is calculated for 8 keV X-ray. The results show that a focused beam size of 5.7 nm and a mean efficiency of 26% can be obtained by a tilted MLL with an outmost zone thickness of 5 nm. MLL is proved theoretically to be an effective method to focus X-ray beam.