Abstract: The molten salt cooled pebble bed reactor is one of the advanced reactors, and it is necessary to estimate the nuclear data uncertainty and consequent K_∞ uncertainty because the coolant material and fuel element material are different from those of well developed light water reactors. This can be achieved by the most often used sensitivity and uncertainty analysis software. The results from single pebble model show that the K_∞ uncertainty due to nuclear data is about 0.6%. The 7Li(n, γ) cross section uncertainty is about 14.7% and K_∞ uncertainty is the largest. The cross section uncertainty of graphite in pebble, C and F in coated fuel particle is just smaller. Because ²³⁵U and ²³⁸U reaction is the most sensitive to K_∞, the corresponding K_∞ uncertainty is second only to 7Li. Therefore, the accuracy of nuclear data needs to be improved in the molten salt cooled pebble bed reactor, especially for the 7Li(n, γ) cross section.