This investigation was made to clarify how the indentation hardness of optical glass depends upon the effect of densification or plastic flow.To examine the effect densification has upon indentation hardness, optical glasses with Knoop indentations were heat-treated at temperatures below their transition temperature. After heat-treatment, the percentage of change in the longer diagonal length of Knoop indentations was measured. These experiments were carried out on fused silica, BK, K, KF, F, SF, SK, SSK, BaF and BaSF glasses. Indentation recovery was found to be very small in optical glasses. Further, it was found that SF glasses which have the largest indentations show little or no change, while BK glasses which have small indentations show a much greater percentage of change. These results show that indentations in optical glasses are not always related to densification.Using an interference microscope, Knoop indentation perimeters were observed in BK, KF, LLF, LF, F, SF, SK and SSK glasses. Without exception, a hump was found to be formed entirely around each indentation. This humping phenomenon shows that plastic flow occurs during indentation. Moreover, it was found that the ratio of the width (c′) of the hump around the shorter diagonal to the shorter diagonal length (a′) is approximately proportional to c/a calculated from Hill's theory on plastic-elastic materials, where (a) is the spherical radius of the cavity generated by a pressure and (c) is the spherical radius of the plastic region around the cavity. This means that Hill's theory can be applied to the indentation phenomenon of optical glass and accordingly the indentation of optical glass is always related to plastic flow.The relationship between Young's modulus and glass composition was found to be almost parallel to the relationship between Knoop hardness and glass composition. Both Knoop hardness and Young's modulus increased with the content of net-work former. This supports Hill's theory. It was also found that c/a decreases with the content of glass net-work former.From these experimental results, it would be concluded that the indentation hardness of optical glass, which contains a large amount of net-work modifier, is far more an expression of resistivity to plastic flow than densification.