Abstract: Abstract: Some sandy sediment in the coasts in Hainan province is rich in organic matters and the sand particles might be coated and disseminated by organic carbon. This could alter the surface of the sand and thus compromises the mechanical strength of the concrete made using such sands as aggregates. This paper experimentally investigated how organic-disseminated sand particles impact the mechanical properties and constitutive equation of the associated concrete. The sample species were prepared by mixing 20% (w/w) and 7.5% of lime with water at water: cement ratio of 0.45; they were then further mixed with sand particles disseminated by organic carbon coating. Concrete using standard sands served as the control. The mechanical properties and constitutive equation of the specimens were measured using the unconfined and uniaxial compressive tests. The unconfined compression test was to measure the compressive strength at curing time of 7d, 14d, 21dand 28d, respectively, and their failure mode. The uniaxial compression test was to measure the stress-strain curve, the stiffness variation rule and the modified Duncan-Zhang constitutive equation. The results show that 1) the failure mode of the concrete with organic-disseminated sands was plastic shear failure and brittle shear failure, while the failure mode of the specimens with standard sand at the same curing time was brittle shear failure. 2) The compressive strength of the concrete with organic- disseminated sands was significantly smaller than that with standard sands at early stage of the curing, although both increased exponentially with the curing time. The compressive strength of each specimen reached asymptotically to a constant after 14 days of curing. 3) The stiffness of the concrete with organic- disseminated sands increased with curing time. After the loading was applied, its shear modulus increased with the axial strain initially, but when the strain reached about 2%, the shear modulus started to decrease with a further increase in strain. In terms of plasticity, the former is known as stiffness hardened and the latter is known as stiffness softening. The stiffness of the concrete with organic-disseminated sands increased with curing time, and the peaked shear modulus at Day 28 is 4.36 times that at Day 7. 4) The stress-strain curve of the concrete with organic-disseminated sands was a interplay of various deformation processes under uniaxial load, and can be divided into two stages: plastic stage and softening stage. 5) The modified Duncan-Zhang model was obtained to describe the full stress-strain curve. Comparison with measured data showed that the model adequately described the stress-strain of the concrete with organic-disseminated sand particles.