The purpose of this study was to compare the compression index value, Cc, of selected backfill materials from Bondo quarry, East Manggarai Regency, East Nusa Tenggara Province, using the correlation of liquid limit and plasticity index. The material was classified as CL or ML based on its plasticity qualities. The association between LL and PI yielded considerably different compression indexes, Cc values. In general, Cc values based on PI correlation are greater than those based on LL correlation, with values of 0.4652 for 5% PI, 0.4673 for 7% PI, 0.4694 for 9% PI, 0.4714 for 11% PI, and 0.4735 for 13% PI (Nakase et al. 1988). The Cc value is based on the LL correlation: 0.2294 for 23% LL, 0.2894 for 29% LL, 0.3494 for 35% LL, and 0.3894 for 38% LL and 0.4194 for 42% LL. Civil engineers must use extreme caution when using Cc values derived from LL and PI correlations. The smallest Cc values for 23%, 29%, 35%, 38%, and 42% LL correlations are 0.0323, 0.0321, 0.0318, 0.0317, and 0.0315, respectively, whereas the biggest Cc values are 0.2294, 0.2894, 0.3494, 0.3894, and 0.4194. The lowest Cc values for 5%, 7%, 9%, 11%, and 13% PI correlations were 0.0663, 0.0928, 0.1193, 0.1458, and 0.1723, respectively, whereas the highest Cc values for the same PI correlations were 0.4652, 0.4673, 0.4694, 0.4714, and 0.4735. The LL and PI correlations yielded dramatically different Cc values, with the PI producing the highest values.