A total of 6,804 records for Duroc breed were collected from three farms registered at the Korean Animal Improvement Association (KAIA) from 1998 to 2004 of which both records from two ultrasound modes (A and B) were analyzed to estimate the variance components of carcass traits. Three carcass traits backfat thickness (bf), loin eye muscle area (Ima) and lean meat percentage (Imp) were measured. These traits were analyzed separately as bfl, Imal and lmp1 for ultrasound mode A and bf2, lma2 and lmp2 for ultrasound mode B with multiple trait animal model by using MTDFREML (Boldman et al., 1993). All the traits revealed medium heritability values. Estimated heritabilities for bfl, bf2, Imal, lma2, lmp1 and lmp2 were 0.45, 0.39, 0.32, 0.25, 0.28 and 0.39, respectively. Estimated genetic correlations for traits bfl and bf2, Imal and lma2, Impi and lmp2 were positive but low. Specifically, genetic correlations between bfl and bf2 was 0.30 while the estimates for lean traits between Imal and lma2 and between Impi and lmp2 were 0.15 and 0.18, respectively. Conversely, high negative genetic correlations existed between bfl and the lean traits lma2, lmp2. Likewise, the estimated genetic correlations between Imal and lma2 and Impi and lmp2 were low.