Atherosclerotic plaques within coronary arteries result in coronary stenosis which compromises blood flow to the distal myocardium and result in local physiological perturbances such as recirculation and shear stress [1]. Coronary lesion severity is often graded by the amount of lumen encroachment or stenosis it causes, and most clinical decisions are made based on lesion stenosis severity [2]. However, other lesion characteristics, such as eccentricity are determinants of both shear rate and the extent of flow recirculation in in-vitro and in computational fluid dynamics (CFD) analysis of the arteries. It has been shown that 81% of the coronary lesions studied by intravascular sonography were eccentric and the eccentric lesions are more likely to be seen in patients with cardiovascular symptoms [3]. In this study, a particle image velocimetry (PIV) method was used to analyse the effect of the eccentricity on blood flow dynamics during a transient flow cycle and also to investigate how the eccentricity impact on the flow behaviour changes as the Reynolds number changes. Two eccentric and concentric stenosis models were manufactured from silicone (R'Tech Co., Ltd, Japan). All models were a box-type with an inner diameter of 4 mm (Fig. 1). The model sizes were based on the values of the human left main artery (LMA) and left anterior descending (LAD) [4]. The resultant models had a 70% diameter stenosis based on NASCET (North American Symptomatic Endarterectomy). The models were connected to a particle image velocimetry (PIV) circuit and an Nd:YAG solid laser (BWN-532-100E, B&W TEK Inc., USA) of wavelength 532 nm and output power 100 mW was used to produce a light sheet of 1 mm thickness in the desired area of visualization. The desired thickness and width of the light sheet were achieved by aligning a micro lens with 105 mm focal length and 2.8 F ratio (Micro-Nikkor, Nikon Co. Ltd., Japan). A pulsatile flow waveform was generated using a screw pump (NBL30PU, R’Tech Co. Ltd., Japan). At different times corresponding to different Reynolds numbers (Re = DU/v, D is the diameter of the parent artery, U is the velocity at the centre of the parent artery and v is the kinematic viscosity), the maximum axial velocity, shear strain rate and the recirculation length were measured for both eccentric and concentric models and compared with each other.