A new side-projected catheter for in vivo blood flow measurements has been developed and tested. This catheter is designed to measure blood flow in both the forward (towards the catheter tip) and reverse (away from the catheter tip) flow directions. It consists of two multimode optical fibres with core diameter of 50 xm and cladding diameter of 125 µm. One fibre transmits the laser beam into blood and the other receives the back-scattered light from the erythrocytes within the probe volume. The experimental plots of the dominant frequency (frequency with the highest amplitude) against the flow velocity are found to be linear in both the forward and reverse flow directions. This result was reaffirmed by the numerical flow simulation along the fibre catheter side wall. In the simulation, it was found that the flow within the boundary layer is indeed laminar, and hence the relationship between the Doppler shift frequencies and the flow velocities is linear, thereby making the linear calibration possible for predicting the free stream flow velocity. The analysis of the experimental and the computational results indicated that the boundary layer along the side wall of the fibre catheter varies with both the flow velocity and flow direction. Furthermore, using the fibre optic catheter with an increasing fibre core separation, it was observed experimentally and numerically that the percentage of the free stream flow velocity increases, suggesting that the projection of the measuring probe volume is indeed moving towards the free stream domain.