Molybdenum disulphide (MoS₂) has emerged as a promising candidate for low-power digital applications. However, grain boundaries play a decisive role in determining the carrier mobility and performance of MoS₂-FETs. In this work, we report a systematic study on the grain boundary of chemical vapor deposition (CVD) MoS₂. We found that in the ON-state, if current flows across a grain boundary that is aligned perpendicular to the channel length, the current of CVD MoS₂-FETs can be significantly reduced, while in the OFF-state, the effect is negligible. Metal-insulator-transition is clearly observed, indicating the high quality of our CVD samples, and it is also shown that grain boundaries increase the metal-insulator-transition crossover-voltage in MoS₂-FETs. Thereby, this work provides useful information and guidance in understanding the nature of carrier transport in synthesized MoS₂ devices, and the developed framework can be applied to other 2D semiconductors in general, as well as in optimizing the CVD process and device design with 2D materials.