Aortocoronary bypass (ACB) surgery is a treatment to bypass a blocked artery using a graft. Approximately 500,000 bypass surgeries are performed in the U.S. each year. Of these, about 15% to 20% have an early-phase failure, typically in the proximal region. A natural question which arises is whether fluid dynamic geometry plays a role in the patency. Since only a few studies have considered proximal region geometry, this motivates the present study. Blood flow in a coronary artery bypass often involves very complex fluid dynamic behavior. Numerical simulation is employed to investigate the flow field environment of the proximal region of a bypass. A series of different branching models with laminar inflow are considered to systematically investigate the geometrical effect under different flow conditions. Model types (with branch diameter to host diameter ratio , with or without blend at the junction) include T-junction and host to branch junction with a radius of curvature with and without helical pitch. Some flow conditions included non-Newtonian blood and non-steady flow. Non-blended T-junction is subjected to flow separation at the inner wall of the branch unless the flow rate ratio is high and is small. By employing a blend radius at the T-junction, there is a critical flow rate ratio where separation near the inner wall of the junction can be diminished under steady state flow condition. This parameter is known as Given a blended T-junction, there is a common branch radius which corresponds to that gives the minimum separation scale under steady state flow condition or delays the onset of separation under non-steady flow conditions. This parameter was found to be independent of . A geometric correlation that corresponds to non-separated flow in blended T-junction was developed and can be expressed as . This correlation is independent to flow rate ratio.