Real-time monitoring is essential for understanding and eventually precise controlling of the growth of two dimensional transition-metal dichalcogenides (2D TMDCs). However, it is very challenging to carry out such kind of studies on chemical vapor deposition (CVD). Here, we report the first real time $in-situ$ study on the CVD growth of the 2D TMDCs. More specifically, CVD growth of molybdenum disulfide (MoS$_2$) monolayer on sapphire substrates has been monitored $in-situ$ using differential transmittance spectroscopy (DTS). The growth of the MoS$_2$ monolayer can be precisely followed by looking at the evolution of the characteristic optical features. Consequently, a strong correlation between the growth rate of MoS$_2$ monolayer and the temperature distribution in the CVD reactor has been revealed. Our result demonstrates the great potential of the real time $in-situ$ optical spectroscopy for the realization of the precisely controlled growth of 2D semiconductor materials.
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