In this study, Hargrove Grindability Index and Impact Strength Index values of coals collected from 17 coal seams in the Zonguldak Hardcoal Basin were determined. Then, the effect of the Impact Strength Index test on grindability was examined by investigating the relationship between Hargrove Grindability Index and Impact Strength Index values. The results indicated that the best relationship between Hargrove Grindability Index and Impact Strength Index values was found to be in the form of a parabolic function, i.e., coals with high values of Hargrove Grindability Index and low values of Impact Strength Index are much easier to grind. Furthermore, investigations have been conducted as to the effect of volatile matter and carbon contents as well as the rank and maceral composition of the samples on Hargrove Grindability Index and Impact Strength Index values. As a result, it has been proven that Impact Strength Index values can be assessed as a criterion of grindability.