Alumina (Al 2 O 3 ) is one of the most important technical ceramic materials due to its good properties and its acceptable price. For some purposes, it is important to achieve either a very smooth or a rough surface of the ceramic object. The aim of this study was to evaluate the influence of different surface finish processes on the surface roughness of alumina and, consequently, on the measured Knoop hardness. Four types of samples were prepared, each with a different surface finish – ground with different SiC abrasive papers or polished with diamond pastes with different granulation. For each sample three indentation loads were used: 0.4903 N (HK 0.05), 4.903 N (HK 0.5) and 49.03 N (HK 5). It has been noticed that all samples exhibited a decrease in hardness with the increasing applied test load, a phenomenon known as the normal indentation size effect (ISE). In order to investigate this occurrence, three mathematical models were applied, i. e., Meyer's law, the proportional specimen resistance model (PSR) and modified proportional specimen resistance model (MPSR). The results showed that the alumina surface roughness significantly affects the measured Knoop hardness. Dependence of Knoop hardness and the indentation load can be quantified by all three mathematical models.