A new dual-fiber probe for in vivo blood flow measurement is designed based on theoretical considerations. The probe is designed to measure blood flow in vivo in both the forward (toward the probe) and reverse (away from the probe) flow directions at flow rates ranging from 0 to 1.5 ms-1. It is comprised of a reflective surface, polished at the angle, attached to the ends of two normally terminated optical fibers. This reflective surface is used to project the probe volume to the sides of the fibers where the fluid boundary layer is expected to be small. It is found that to project the probe volume out of the cavity formed between the fiber tips and the reflective surface in the forward direction, the reflective surface angle should be between 23 to 40 deg. The new dualfiber probe with the reflective surface at 25 deg to the fiber axis was fabricated and tested. In the forward and reverse flow directions, a linear relationship exists between the flow velocity and the Doppler shift frequency obtained even though the probe volume is still within the boundary layer adjacent to the sides of the fibers. This suggests that flow within the boundary layer up to a velocity of 1.0 m s-1 is laminar and is fairly independent of the flow velocity and flow directions. The new dual-fiber probe holds promise for applications in the clinical context.