The purpose of this study was to develop a working fluid with the proper viscosity to mimic blood flow to evaluate an ablation catheter. We proposed a working fluid that matches the blood viscosity, using glycerol-saline solution or xanthan gum (XG)-saline solution. We compared the two solution mixtures at various concentrations (for glycerol-saline solution, 10%, 30%, and 50%; for XG-saline solution, 0.10%, 0.11%, 0.12%, and 0.13%) by measuring their temperature dependence on viscosity. To test the developed working fluids, RF ablations were performed with both solutions, and the flow around the catheter during ablation was observed by particle image velocimetry (PIV) method. In addition, to test the efficacy of the solution mixtures, myocardium was observed after it was soaked separately in the two solution mixtures. The viscosity of both kinds of solution mixtures increased at higher concentrations and decreased with increasing temperatures. At 37°C, the viscosities of 50% glycerol-saline solution and 0.11%-0.13% XG-saline solution were 3-4 mPa s, a value that is similar to the viscosity of blood. The upward flow resulting from heat convection by ablation was observed around the catheter in the PIV analysis using an in vitro model without flow. The solution mixture with the higher viscosity flowed at a lower velocity around the catheter, and the myocardium soaked in 50% glycerol-saline solution shrunk into a black and tough tissue. On the other hand, the myocardium soaked in XG-saline solution showed no change in color or shape. In conclusion, the XG-saline solution is a superior option for evaluating ablation catheters.