Wire sweep, the deflection of wires during the mold flling stage, is an important phenomenon in IC packaging process. Various methodologies of wire sweep analysis have been introduced to get better prediction and match the experimental measurements by many researchers. Typical wire sweep analysis method grabs results of mold flow simulation to calculate the drag force applied on the wire and the amount of wire sweep. In the mold flow simulation, however, only the volume of epoxy molding compound is constructed and the wires are neglected because of the modeling complexit. In this way, i the wires are ver closed, the local flow field around the wire will be different That is, each wire can afect the downstream flow feld, the drag force magnitude of the downstream wires, even the wire sweep of all downstream wires and the wire sweep result will be wrong. This is the so-called wire densit effect. In order to know more about wire density effect a CFD software-FIDAP is used to perform several fow simulations: flow between two parallel plates with cylinder(s) inside and then the downstream velocit distribution can be investigated. According to the trend of downstream velocity recovery rate, the wire density effect can be observed. The faster the downstream velocity recover rate, the effect of upstream cylinder on downstream cylinder is smaller and then the wire densit effect is less signifcant. After a series of simulations, three conclusions can be drawn. First, since each flow around the wire will be reorganized, the velocity feld change caused by a wire only affect the next one downstream wire and doesn't extend to the other downstream wires. Second, the mold cavity thickness, or the mold cavity thickness/wire diameter ratio, plays a strong role in the determination of wire density effect importance. When the ratio is small, the wire density effect is less important. In contrast, when the ratio grows, the wire density effect should be considered for better wire sweep prediction. Third, the position of wire in the mold cavity thickness direction also affects the wire density effect but it's less signifcant compared with the mold cavity thickness.