The objective of this study was to evaluate the effect of laser fluence, repetition rate and catheter size on the ablation of thrombus by an excimer laser, in-vitro. Ablation of reconstituted thrombus was conducted with a XeCl (308 nm) excimer laser. Fluences of 30 and 45 mJ/mm2, repetition rates of 25 and 40 Hz, and catheters of sizes 0.9, 1.4, and 2.0 mm were used. Ablation efficiency (?g/J*pulse), and mass removal rate (?g/sec) were calculated by measuring the pre- and post-ablation weight of clot blocks (~2x1x1 cm3) placed on a precision scale. Depending on lasing parameters and catheter size, the ablation efficiency ranged from 0.51+/-0.09 to 1.13+/-0.20 ?g/J*pulse (N=12), while the mass removal rate ranged from 105+/-23 to 936+/-146 ?g/sec (N=12). Overall, there was a strong dependence of both quantities on the catheter size. Ablation efficiency was not significantly affected by either the fluence or the repetition rate, while mass removal rate increased with increased fluence and repetition rate. Excimer laser can effectively ablate thrombus in vitro. Catheter size affects both ablation efficiency and mass removal rate, while fluence and repetition rate mainly affect the mass removal rate.