Microbial-Induced Carbonate Precipitation (MICP) has been explored for more than a decade as a promising soil improvement technique. However, it is still challenging to predict and control the growth rate and characteristics of CaCO3 precipitates, which directly affect the engineering performance of MICP-treated soils. In this study, we employ a microfluidics-based pore-scale model to observe the effect of bacterial density on the growth rate and characteristics of CaCO3 precipitates during MICP processes occurring at the sand particle scale. Results show that the precipitation rate of CaCO3 increases with bacterial density in the range between 0.6×108 and 5.2×108 cells/ml. Bacterial density also affects both the size and number of CaCO3 crystals. A low bacterial density of 0.6×108 cells/ml produced 1.1×106 crystals/ml with an average crystal volume of 8,000 µm3, whereas a high bacterial density of 5.2×108 cells/ml resulted in more crystals (2.0×107 crystals/ml) but with a smaller average crystal volume of 450 µm3. The produced CaCO3 crystals were stable when the bacterial density was 0.6×108 cells/ml. When the bacterial density was 4-10 times higher, the crystals were first unstable and then transformed into more stable CaCO3 crystals. This suggests that bacterial density should be an important consideration in the design of MICP protocols.