In this study, the petrographic characteristics affecting the strength of granitic rocks were investigated and a new mineralogical-based model was suggested for the prediction of uniaxial compressive strength (UCS) in line with the obtained data. Increase in quartz and alkali feldspar contents with decrease in plagioclase content partially increase the strength of granitic rocks. However, the results of modal analyses show that the rate of physically competent minerals do not directly affect strength of the granitic rock because the mineral size has a greater effect on strength than that of mineral type. The higher strength values were determined for the rocks containing monotype mineral in terms of size and quantity. In the scope of the study, significant relationships with a 0.89 correlation coefficient between UCS values and the Quality Index values (QI) were determined using mineral percentages, content and size. Because the modal analysis technique is a time-consuming process, ultrasonic velocity ratio (UVR) was defined in order to evaluate rock strength in the study and significant relationships were found with the correlation coefficient of 0.77 between UVR and QI. To estimate UCS using only ultrasonic wave velocity, it was determined whether there is a statistical relationship between UVR and UCS. The test result shows significant relation was found characterized with correlation coefficient of 0.84 value. UCS values obtained from the proposed equation and experimental studies were compared to test the suggested model; the results of the study showed that UCS values can be predicted without needing modal analysis with the new method proposed.