Purpose: To investigate whether toothpastes with diamond powder vs those with traditional abrasives abrade dentin and enamel differently and to determine the relative dentin abrasivity (RDA) and relative enamel abrasivity (REA) values of those toothpastes. Materials and Methods: Dentin and enamel samples of bovine permanent incisors were randomly allocated into groups of eight, brushed with 20 different toothpastes (three of which contained diamond powder) and analysed for their RDA and REA values. Results: Toothpastes with diamond powder exhibit low RDA values but high REA values. Some RDA values exceeded the ones declared by the manufacturer. Conclusion: Diamond powder as an abrasive might have a mild action on dentin, but it is highly abrasive on enamel. Keywords: abrasivity, diamond powder, RDA, REA, toothpastes