A study on how correlation among FWD deflection data influences accuracy of the backcalculated pavement layer moduli is presented. FWD data were obtained from the Ministry of Construction, Road Test Section 609 and the data with stronger correlation were used in this study. Deflections without correlation and with similar correlation to the FWD data were generated using Monte Carlo simulation in order to investigate the effect of correlation on the backcalculation results. Furthermore, backcalculation analysis using theoretical deflection data with the same standard deviation but different correlations showed smaller variation in the backcalculation results for the deflection data with stronger correlation.