Abstract: The nuclear data are the basic data for reactor physics calculation. It is significant to study the contribution of their uncertainty to the uncertainty of reactor physics calculation for improving the safety and economy of reactor. Based on the ENDF/B-Ⅶ.1 library, the covariance library including the variance and covariance for all the basic cross sections and all the groups was generated using the NJOY code. The uncertainty analysis code UNICORN for reactor physics calculation was developed based on the statistical sampling method. Based on the UNICORN and covariance library, the uncertainties of k_∞ for Three Mile Island (TMI) lattice and RB31 benchmark, due to the uncertainty of basic cross sections, were analyzed. The results indicate that the uncertainty of ²³⁸U(n, γ) cross section is the largest uncertainty to the k_∞ of TMI lattice and the relative uncertainty to the k_∞ is up to about 0.4%, and different sources of covariance library effect the uncertainty of analysis result.