Rock cutting with an abrasive waterjet system has been attempted in mining and construction fields. In abrasive waterjet cutting, the overall operating cost depends on the cost of the consumed abrasive. For this reason, it is important to identify an optimum abrasive feed rate for cost and performance efficiency. In this study, the concept of the abrasive/water ratio in volume (i.e., mixing rate of the abrasive and water amount) was introduced to explore the optimum feed rate for the cutting performance according to the water flow rate. Rock cutting tests were performed on intact granite specimens at different abrasive/water ratios. The test results were analyzed based on the effect of the abrasive feed rate in a constant water flow rate condition and on the effect of the water flow rate in a constant abrasive feed rate condition, respectively. In addition, the ratio of the abrasive particle size to the abrasive-water mixture flow space (i.e., mixing tube size) was considered and analyzed to determine the optimum abrasive/water ratio. Under the conditions given in this study, the optimum abrasive/water ratio ranges from 0.08 to 0.20 and tends to increase with an increase in the average particle size, particle size uniformity of the abrasive, and mixing tube size. In the performance analysis of rock cutting, a higher water flow rate causes a better cutting efficiency; meanwhile, the optimum abrasive feed rate achieves the maximum cutting performance. These findings can help to estimate the amount of abrasive required for efficient rock cutting.