This work investigates semibatch precipitation of magnesium carbonate at ambient temperature and pressure using Mg(OH)2 and CO2 as starting materials. A thermal analysis method was developed that reflects the dissolution rate of Mg(OH)2 and the formation of magnesium carbonate. The method was successfully used to determine the composition of solids precipitated from the reaction. Concentration profiles of Mg2+ and total carbon over time were determined from the liquid phase. The influence of CO2 flow rate and stirring rate on precipitation was analyzed based on comprehensive information from the liquid and solid phases. A stirring rate of 650 rpm was found to be the optimum speed as the flow rate of CO2 was 1 L/min. Precipitation rate increased with gas flow rate, which indicates that mass transfer of CO2 plays a critical role in this precipitation case. Magnesium carbonate trihydrate (nesquehonite) was formed as a bunch of needle-like primary crystals.