Abstract: The uncertainty of nuclear cross-section is the major factor causing the uncertainty of k_eff . The direct Monte Carlo method was used to calculate the uncertainty of k_eff caused by uncertainty of nuclear cross-section data. The method simulated the distribution of nuclear data firstly, then used the data for integral parameters calculation with core calculation program and used statistical method to get uncertainty of k_eff . Two -different kinds of nuclear devices, Jezebel-²³⁹Pu and China Experimental Fast Reactor, were calculated using this method and the results show that the direct Monte Carlo method for uncertainty analysis is reasonable and achievable.