Microbial-Induced Calcium Carbonate (CaCO₃ ) Precipitation (MICP) has been explored for its potential engineering applications such as soil stabilization, but our understanding of the fundamental MICP processes at the microscale is limited. In this study, real-time in situ micro-scale experiments were conducted using glass slides and microfluidic chips (synthetic porous media which simulate soil matrices to model the conditions similar to actual MICP treatments) to visualize the CaCO₃ precipitation process. The results of this study show that irregularly-shaped CaCO₃ precipitates initially emerged on bacterial aggregates and subsequently dissolved with time as regularly-shaped CaCO₃ crystals started growing; less stable and smaller CaCO₃ crystals may dissolve at the expense of growth of more stable and larger CaCO₃ crystals. The time-dependent phase transformation of CaCO₃ precipitates makes the size of the crystals formed during MICP highly dependent on the time interval between cementation solution injections during a staged injection procedure. When the injection interval was 3-5 hours, a larger number of crystals (200-1000 per 10⁶ μm³) with smaller sizes (5-10 μm) was produced. When the injection interval was longer (23-25 hours), the crystals were larger (10-80 μm) and fewer in number (5-20 per 10⁶ μm³). The direct observation of MICP processes in this study improves the understanding of MICP fundamentals and the effect of MICP processes on the properties of CaCO₃ crystals formed after MICP treatment. These observations will therefore be useful for designing future MICP treatment protocols which improve the properties and sustainability of MICP-treated samples.