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　of238U(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.　©　2015,　Editorial　Board　of　Atomic　Energy　Science　and　Technology.　All　right　reserved.