Nuclear density cone penetrometer(ND-CPT), incorporating the nuclear density probe into the probe of piezocone penetrometer, is a newly developed in-situ testing device that can simultaneously measure cone resistance, sleeve friction, pore water pressure and density. According to the principle that ND-CPT measures the average wet density of the soil within a spheroid volume around the central point of the source and the detector, the backscatter and average value models which can calculate the average density of soil within a spheroid volume are firstly proposed. Then, assuming ND-CPT is penetrated into various heterogeneous strata, the differences between the average density profiles and the actual density profiles are investigated; and the results show that the measured values are considerably different from the actual values near the boundaries of strata. Finally, the measured profiles of seabed sediments in Ariake Sea, Japan, are interpreted and the actual profiles are deduced. The research results can provide the basis for accurate soil classification using ND-CPT.