The velocity profiles in the core of a pure viscoelastic liquid displacing a Newtonian oil in a 500 µm microchannel were measured with micro-particle image velocimetry (µPIV) to investigate the onset of elastic turbulence. During displacement, a film of the displaced phase remains on the channel wall while interfacial instabilities develop upstream of the displacing core. Two displacing phase flow rates were investigated at 0.1 ml/min and 0.2 ml/min, corresponding to elastic Mach numbers, Ma, of 0.43 and 0.86 respectively. It was found that the velocity profiles in the streamwise and wall-normal direction are almost symmetric with respect to the centre of the core phase, apart from the case at high Ma and at the position of maximum interface deformation. Velocity fluctuations are present for 0.2 ml/min while they are almost zero for 0.1 ml/min, indicating the presence of elastic turbulence at large flow rate. Similarly, the calculated turbulent kinetic energy terms and Reynolds stresses have significant values at 0.2 ml/min but are almost zero at the low flow rate. In both flow rates, a circulation pattern is formed below the interfacial instability.