This study investigates variation of flow characteristics due to variation of branch channel width and discharge ratio at bifurcation channel using 2D numerical model. The calculated result considering secondary flow is more accurate and stable than without considering one. The diversion flow rate (Q3/Q1) is reduced by flow stagnation effect according to the interaction of the secondary flow and flow separation zone in branch channel. The less upstream inflow or the lower upstream velocity, the bigger variation of diversion flow rate by changing branch channel width. At uniform downstream boundary condition, the rate of change in Froude number of downstream of main channel(Fr2)-diversion flow rate (Q3/Q1) relations is similar about -2.4843~-2.6675 when branch channel width ratio (b/B) is decreased. At uniform diversion flow rate (Q3/Q1) condition, the width of recirculation zone in branch channel is decreased when branch channel width ratio (b/B) is decreased. The less upstream inflow in the case of increasing branch channel width or the narrower branch channel width in the case of increasing upstream inflow, the bigger reduction ratio of recirculation zone width. At uniform inflow discharge (Q1) condition, diversion flow rate, the width and length of recirculation zone in branch channel are decreased when branch channel width ratio (b/B) is decreased.