A Molded Case Circuit Breaker(MCCB) is core device of electric distribution sytem. The performance of the MCCB depends on how quickly quenching of the arc between electric contacts. Inside the MCCB, an arc grid is used to separate arcing into the arc to isolate. Generally, the characteristics of the arc quenching performance by the arc grid are influenced by material, numbers, and shape. In this thesis, a numerical analysis of the flow rate inside the MCCB is interpreted to conduct a study of internal flow in the shape of the arc of the arc grating to predict the inside flow. The arc extinction function of the arc grid located inside the MCCB quenching room is critical to the arc movement of the arc. Thus, a numerical analysis of the arc characteristics of the arc grid resulted in a numerical analysis of the conditions in which the arc produced by the electrodes can move rapidly into the arc grid.