The use of helix geometry for arterial grafts has been proposed on the hypothesis that by intentionally inducing swirling or spiral flow in the grafts, hemodynamic performance of the grafts might be improved. To investigate their hemodynamic performance, the present study numerically simulated the flows in the helical grafts, not only for comparison with conventional grafts but for their parameter analysis of Dean Number, helical pitch, and amplitude. Results showed that the helical graft achieved three dimensionality swirling flow, more uniformly distributed flow field and high wall shear stress (WSS) which continued in the straight part of the graft downstream. However, increased pressure drop was predicted in helical graft; flow areas with low velocity will concentrate in one corner, which might possibly suffer from flow stagnation and inhabitation, leading to a possible vulnerability to hemodynamic failure, intimal hyperplasia (IH) and thrombosis. The parameter study indicates that even at the same Dean number but with different geometry, the hemodynamic performance of two grafts are totally different. Shorter helical pitch and larger helical amplitude do improve the graft's hemodynamic performance, but may not be mechanically robust or applicable clinically. The current study increases fundamental understanding of the flow mechanism in swirling flow grafts.